Sleep, Jet Lag & Circadian Rhythm

How does it all work? My current mental model.

Aug 05, 2024

This post isn’t about fat loss. If your sleep is fine, you can skip it. Terrible sleep will often affect your health and can cause fat gain/prohibit fat loss, but this is more of an academic explanation of my paradigm & understanding, not focused on actionable advice.

Edit: This baby is now clocking in at over ~~4,000~~ ~~6,100~~ ~~9,000~~ 10,000 words, ~~one of my longer posts~~ by far my longest post ever. I guess my goal here was to write down a brain dump of all my knowledge about circadian rhythms. It’s a vastly understudied and under-understood topic, and the more people at least understand the basics, the better.

Kinda like you should probably know that peanut allergies and celiac exist, in case you ever react really badly to something, or understand how diabetes works, it’s good to know the basics of how our circadian rhythm works.

If you finish this post, you’ll know more about the circadian rhythm than pretty much anyone out there. Feel free to come back to it later, if you ever have trouble with DST, jet lag, or sleep in general.

Why are you such a sleep nerd?

If you didn’t know, I have a Circadian Rhythm Disorder called Non-24 sleep/wake disorder (Non-24 or N24 for short) which I have put in remission using a ketogenic diet for almost 9 years now. That’s one of the reasons I’m so adamant on staying in ketosis; I can’t work a regular job or have a social life if I don’t.

Why sleep?

Sleep is pretty good. As a mammal, you should sleep a sufficient amount of time, and, importantly, at the correct time. Good sleep is a requirement for good health; I’d put it even above nutrition.

You can go a few seconds to minutes without breathing.
You can go 1-3 days without drinking.
You can go 1-3 days without sleeping.
You can go weeks to months without eating. It’ll suck and is unhealthy, but you’ll live.

I’d say sleep is tied for #2 with drinking in terms of mammalian needs. Lab rodents systematically prevented from sleeping will reliably die very quickly.

If you’re one of those people who thinks sleep is dumb and you hate sleeping and try to do as little as possible of it, this post is not going to try and convince you otherwise.

Sleep Hygiene & Warm Milk

This post is also not about those little tricks & hacks people like to quote about sleep.

You need to go to bed at the same time every day!
Use your bedroom just for sleeping, not for anything else!
Put up blackout curtains to turn your bedroom completely dark!
Just drink some warm milk before going to bed!
Just take a hot shower before going to bed!
Just stay in bed and count sheep even if you’re not tired!
Your bedroom needs to be at exactly 68°F for optimum sleep!

I think most of these are pretty unimportant. Some might help on the margin; clearly it’s more comfortable to sleep at 68°F than at 100°F, you shouldn’t be physically uncomfortable while trying to fall asleep. But I don’t think there’s much of a difference once you’re in the general “comfortable” range.

Similarly, your bedroom should be “dark enough” for you that the light isn’t keeping you awake. I’ve slept in hotels with big floodlights pointed directly at my window, and was unable to sleep without a sleep mask over my eyes. But if there’s a little bit of ambient light coming in, that’s not a big deal. People used to sleep under a full moon all the time, or next to a campfire. If you close your eyes and it’s dark, that’s dark enough.

A lot of the other tricks assume that sleep is just some kind of “habit.” I don’t believe in habits.

Many people also claim that you can train yourself to “become an early riser” or otherwise dramatically change your circadian rhythm. This is false; your circadian rhythm, and therefore biological chronotype, is genetic. You can shift it 1-2h in either direction, but that’s about it.

I think these hacks are marginal at best, and they’re just as useless in the pursuit of good sleep as “skip Starbucks” is to becoming rich or “drink skim milk” is for fat loss.

We need to attack the big, fat elephant in the room:

What the heck is a Circadian Rhythm?

“Circa” means about/around, and “dia” means day. Your Circadian Rhythm is the rhythm your body follows in the span of one day. It is also often called your “biological clock” because that’s basically what it does - keep time in your body.

Your circadian rhythm is what makes you tired, and what wakes you up.

Not just your body, in fact: all mammals have a circadian rhythm, as do all plants, and even plankton. It has been speculated that having a circadian rhythm is one of the first features of what we might call “life.”

That is because all life on earth is highly dependent on the sun, and the circadian rhythm is an adaptation to dealing with the sun.

Plankton rise up to the surface to absorb sunlight during the day, and sink down to avoid predation at night.

Plants orient themselves towards the sun to maximize sunlight exposure for photosynthesis.

Animals evolve into distinct chronological niches that can be quite complex. Some are diurnal (active during the day). Some are nocturnal (active at night). Some are most active around dusk or dawn.

A lot of these are interdependent: if you’re a predatory animal, you need to optimize your chances of catching and eating your prey animals. If you’re a prey animal, you need to optimize your chances of eating and drinking without being eaten by the predators. There’s a sort of evolutionary arms race going on at all times.

If an animal (or plant!) is diurnal or nocturnal, i.e. when it is awake and when it sleeps, is dictated by its circadian rhythm.

Hamsters and mice aren’t awake at night because of really bad sleep hygiene and blackout curtains and cooled sleeping pads. It’s their genetics that makes them nocturnal.

The Circadian Rhythm works via melatonin

Hormones regulate the circadian rhythm. The big one is melatonin, a name you might have heard. It’s a common over-the-counter supplement marketed for helping you fall asleep. And it can do that, but its effects are tricky - especially if you don’t understand how the circadian rhythm works. Just slamming some melatonin from the pharmacy probably will likely do more harm than good.

Melatonin was only discovered in 1958, so it’s not surprising that a lot of knowledge about sleep and the role of the circadian rhythm is relatively new and hasn’t spread that widely yet.

Let’s quote Wikipedia on Melatonin for a second:

[Melatonin] was later identified as a hormone secreted in the brain during the night, playing a crucial role in regulating the sleep-wake cycle, also known as the circadian rhythm, in vertebrates.
[..]
In vertebrates, melatonin's functions extend to synchronizing sleep-wake cycles, encompassing sleep-wake timing and blood pressure regulation, as well as controlling seasonal rhythmicity (circannual cycle), which includes reproduction, fattening, molting, and hibernation. Its effects are mediated through the activation of melatonin receptors and its role as an antioxidant.
[..]
The mitochondria, key organelles within cells, are the main producers of antioxidant melatonin, underscoring the molecule's "ancient origins" and its fundamental role in protecting the earliest cells from reactive oxygen species.

Reactive oxygen species! Will you look at that..

Melatonin gets released in the evening, when the sun goes down. Levels rise continually until they hit a peak, about halfway through your sleep cycle. They drop rapidly after that, and when melatonin gets back to daytime-levels, you wake up. Yay!

Melatonin levels during the circadian rhythm. From https://www.researchgate.net/figure/Melatonin-production-throughout-the-day-Modified-from-10_fig1_363887736

Melatonin Dosage

While we’re at it: you can buy melatonin over the counter (i.e., without a prescription) in many places, including the U.S. Amazon carries many brands. Be careful though, because many of the OTC tablets are dosed way too high for most people.

Here’s a graph from the excellent post Melatonin: Less is sometimes More.

You can see that the endogenous melatonin level (i.e. what your body would naturally produce if all were right) only goes up to about 50pg/mL. In this study, they gave people different dosages of melatonin and then measured their serum levels. The 10mg dose produced such high serum levels that the people were still sky-high in the morning (and probably rest of the day) after taking it. That’s way too much!

If your melatonin is high in the morning, your body won’t get ready for the day. You’ll feel exhausted and your circadian rhythm won’t be able to entrain properly. It might have the opposite effect you intended.

They also tested a 0.5mg dose, which is on the low end of what you can find in OTC supplements. The curve comes down in alignment with the normal, endogenous melatonin levels you would expect, and so is a much better fit.

I can personally confirm this: I tried even 3mg of melatonin and it knocked me out. I woke up 12h later feeling like hell and wanting to go back to sleep. Definitely a no-go.

On the other hand, I used 0.5mg tablets for Daylight Savings Time this year, and they worked great. 0.5mg seems like a decent dosage for me personally, but if you try melatonin for yourself, you might have to experiment. Just be aware that the dose makes a HUGE difference and more is most likely worse. You can often just cut the tablets in half or quarters with a knife if you want to play around with doses.

For comparison, I just typed “melatonin” into Amazon and here are the dosages per tablet of the first results that came up:

12mg (lol)
5mg
5mg (“Extra Strength” as if that was a bonus lol)
10mg
3mg

I wouldn’t use any of these, for anything.

Personally I used a brand called “Life Extension” but if you just search for “melatonin 0.5mg” you’ll find plenty of options. There’s also time-release melatonin, which might be interesting. I haven’t tried it.

Chronotypes: Larks & Owls

It’s quite obvious that some people are just natural morning people, and others are not. That’s so trivial to observe there are many stereotypes and nicknames about it, “night owl” being the most common one. I don’t know I’ve ever heard someone casually mention “lark” or “morning lark,” but that’s apparently the counter example.

Larks are much more rare than night owls. There are 2 reasons for this: one is genetic, the other is our modern age, on which more later.

Yes, your chronotype is largely genetic. You can shift it around a little bit, but if you were born a night owl, you will never become a true lark.

You can totally wake up in the very early morning using an alarm every day for the rest of your life, but you cannot move your circadian rhythm very much. That’s just not how the biology works.

Sleeping & waking up at times not appropriate for your circadian biology can be pretty bad for your health. Some people do ok doing this most of their lives, many don’t. In fact, most probably don’t.

Remember when I said that mammals need to sleep not only a sufficient amount, but also at the right time? That’s where your chronotype comes in. There’s a reason you probably can’t fall asleep at noon and sleep for 8 hours straight; and it’s not just that you’ve only been awake for half a day at that point: your circadian rhythm is set up differently.

You will only naturally become sleepy when your circadian rhythm dictates it by raising your melatonin. And you will only naturally wake up (absent alarms/saber tooth tigers) when your melatonin has come down enough.

Chronotypes follow almost a normal distribution, with the exception that there is a long tail for night owls, but not for morning larks:

From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479630/

Yes, there are significantly more extreme night owls than similar morning larks.

I’m a medium to strong night owl personally, but not in the extreme long-tail end of the spectrum. I tend to shift between going to bed around midnight to 2am, and wake up (if there’s no alarm) around 8-10am. That puts my mid-sleep from 4-6am, putting me solidly into the “owl” third of the normal distribution.

Rare Larks

You probably know tons of people who are night owls. But do you know an extreme lark?

I once dated a girl who was such a lark, it was practically a social disability. She’d go to bed at 8pm and wake up before 4am.

As you can guess, we were incompatible in one of the most fundamental biological ways. By the time I got off work, she was getting ready for bed.

Shame, she was a nice gal.

8pm-4am puts her mid-sleep at midnight, the very earliest part of the statistical distribution in the above chart. That’s the lark equivalent of my 2am-10am sleep on the owl side.

Yet I am not nearly the most extreme owl I know: at least 3 people I’ve met, including close friends, are way beyond me. And the graph above shows that this is common: there are way more owls than larks, and they have a much longer tail.

I once knew a guy who would wake up in the afternoon, hang out with friends, party all night, and then start work at 2am or so. (He worked remotely from his laptop.) Then he’d eat breakfast and go to bed.

Roar, Tigers!

Why is there such variation in chronotypes among humans? Speculating, one reason could be that this would allow “shift work” in tribes of hunter gatherers, which would have been an evolutionary advantage.

If your tribe contains a few larks and a few night owls, there would always have been somebody to tend to the fire, or to ring the alarm if a tiger or mammoth or an enemy tribe showed up in the middle of the night.

Were people to all go to bed at 10pm and wake up at 6am like clockwork, you might wake up one morning and find that you and everybody else in your tribe had been killed by a saber tooth tiger after the fire went out.

Our Chronotype Shifts as we Age

Another phenomenon so obvious it’s a cliche: teenagers wake up late and stay up late, whereas old people are extreme morning people.

From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479630/. Red is women, blue is men.

This is actually a function of biology, because our circadian rhythm shifts quite dramatically throughout our lives. The peak, when we sleep/wake the latest and are at our most “owliest,” is just under 20 years of age. Teenagers.

Then we experience a rapid “drop” and our rhythm tends to shift earlier between 20 and 30 years of age, becoming more and more larky.

This creates the “hilarious” mismatch of school children vs. school teachers: the kids aren’t tired until well after midnight, at the “owl peak” of their lives. The teachers are typically at least 10 years older, already having moved halfway down the curve, wanting to go to bed & wake up an hour and a half earlier.

If they’re old teachers, they might be even further along, and might run a whopping 2 hours earlier.

You can also see there’s a slight discrepancy between men and women. Although both experience the same teenaged owl-peak and then a steady drop, men have higher variance in both directions. They’re owlier owls and then become larkier larks starting in their 50s, while it’s pretty similar in between.

Entrainment: Circadian Rhythm is a Function of Sun, to a Degree

The average human being has a circadian rhythm of about 24.5h. The earth’s day is almost exactly 24h long. So how come everyone isn’t shifting by half an hour per day?

Let us remember the plankton & plants: the circadian rhythm evolved so that plants & animals can make best use of the #1 resource in our solar system: the sun.

It will thus come as no surprise that by far the #1 influence on our circadian rhythm, besides our genetics, is sunlight.

All Cavemen have Non-24

Nearly all. They’ve done experiments on this, first with people living in actual caves, later with lab apartments lacking any access to sunlight, or other “timegivers.”

People who live in these conditions begin rapidly losing what’s called “entrainment” to the earth’s day, which is marked by the day/night cycle.

It’s a little bit like playing in an orchestra. If you’re in the same room as the conductor and everybody else, you’ll find it easy to keep the rhythm. Should you mess up, it’s easy enough to get back in (“entrain”).

An orchestra or band without a conductor might work together, or might become very chaotic.

But now imagine that you start out playing in the same room as everyone else. Then you get up and start walking, and you walk out of the opera house and into a sound-proof room, continuing to play your instrument the entire time. You can’t hear the other musicians. You can’t see the conductor.

After a few minutes or even hours of this, how likely is it that you’ll still be playing in rhythm with the other musicians? Extremely unlikely.

It’s the same in those cave/simulated-cave-apartment experiments. Our master “timegiver” is the sun, and our entire society is structured around it. In absence of seeing the sunlight in person, you can follow the cues of other people, or look at a clock. Those are proxies. But if you have nothing, your rhythm will dishevel very quickly.

People in these lab experiments start experiencing very chaotic circadian rhythms. Some of them stay up for a very long time, and then sleep a very long time, without even noticing it (they can’t have clocks, so they can’t tell). Others just seem to sleep and wake up at near random times.

What happens: without a steady “timegiver” to entrain your circadian rhythm to the earth’s 24h day, your true rhythm will come through.

Should that happen to be exactly 24h, then you wouldn’t even see a difference. Very few people have a rhythm <24h, probably that lark I dated. Almost everyone has a cycle slightly longer than 24h, with the average being 24.5h.

If your rhythm is exactly 24.5h long, and you did the cave experiment, you’d probably start experiencing slightly longer “days” and “nights” and would start slightly shifting every day.

But some people are extreme outliers in terms of their genetic circadian rhythm, with cycles of 26h or even longer (or, rarely, significantly less than 24h).

These people have what are called “Circadian Rhythm Disorders,” more on those later.

SUN SUN SUN SUN SUN

If you take only a single thing from this post, it should be this:

GET YOUR MORNING SUNLIGHT ON, EVEN IF JUST FOR 15 MINUTES

Yes, it matters that much. In fact, if you take only 1 thing from THIS ENTIRE BLOG, take that.

Everything is downstream from sleep. Sleep is downstream from circadian rhythm.

Oh yea, DO NOT do shift work ever, especially not for years or decades at a time. You might as well pick up smoking & alcoholism in terms of health.

You do not need to spend tons of time in the sun. And the time of sun exposure matters. Or, technically, the first sun exposure of the day matters the most, and then in the evening the (lack of) last exposure.

Since almost everybody’s rhythm is longer than 24.5h, almost everybody would run slightly too long, and needs to compress the rhythm a little bit each day.

The way to compress your rhythm (“advance your phase” in chrono speak, vs. “delaying your phase”) is by getting early sunlight exposure.

Technically, the way to stretch your rhythm (“delay your phase”) would be to get very late sunlight exposure. But due to the nature of the earth and the sun and all that, it’s impossible in most places to get sun exposure late enough. Unless you live in the arctic in the summertime, you’re gonna have a hard time getting sunlight at 2am.

Remember, even the earliest larks go to bed maybe around 8pm. The sun has probably set in a lot of places at 8pm. You might get a little bit more sunlight in other places, but not for very long. Again, the arctic being the exception. Or space.

To speculate on the reason for this: the onset of melatonin release is the BEGINNING of darkness/light dimming. It is not a normal distribution around the beginning of darkness. Instead, the whole distribution is shifted to the “right” (=later) with the earliest larks getting tired pretty much as soon as the sun sets. The reason is probably that, well, you can’t react to darkness if it hasn’t happened yet.

In short, because almost everybody has a cycle of greater than 24h, it is necessary for almost everybody to compress or “advance” the phase a little bit every day.

Luckily, advancing your phase is as easy as getting a bit of solid sunlight in your eyes in the morning.

Biological components

One interesting tidbit: scientists only very recently (1990s?) discovered the physical receptors in our eyes that actually facilitate entrainment to sunlight.

Before that, it was believed that the regular rods & cones that we see with are responsible for somehow telling the brain that it’s “light” and to entrain our internal clock.

But they did experiments with mice and found that even mice completely lacking rods or cones could entrain to sunlight. This led them to the discovery that it’s an entirely different photoreceptor, with the sole purpose of sending a signal to the Suprachiasmatic Nucleus (SCN), an ancient part of our brain that is the master clock in the body.

My personal ritual: reading in the sun

You don’t have to tan for hours for this effect, I’ve seen as much as 5-10 minutes make a difference. My personal strategy is to read a little bit in the sun every morning, before I even take a shower.

I wake up, use the bathroom, weigh myself on the bathroom scale, and then I make coffee and sit outside in direct sunlight.

It’s usually not that hot yet even in the summer, and in any case, I’ll take a shower when I go back inside anyway.

I’ll do some light reading, nothing crazy. The main goal is to keep me outside for 30 minutes or so. I don’t know if there’s any research on what the optimal response curve is, but “morning” and “15-30 minutes” is a great idea.

Getting sunlight at noon is better than not getting any sunlight, but it’s not going to compress your phase as much as getting the same amount at 9am. Getting 5 minutes is worse than getting 15 or even 30 minutes, but it’s better than nothing.

I don’t know if there’s a benefit, to most people, of more than 30 minutes soon after waking up. Maybe if you have an extremely long cycle of 27h+, which is basically DSPS (a Circadian Rhythm Disorder, more later.)

If 30 minutes in the morning is good enough for me, a pretty strong night owl, most people will be fine with less.

This explains why most people don’t need to consciously manage this at all. If you spend any time outside AT ALL before noon, you’ll probably entrain automatically unless you’re a night owl.

But if you’re a nightowl like me, and you’ve struggled with waking up/falling asleep your whole life, consider using this strategy. I’d consider it important enough, in the long term, to move to a sunnier place where you can easily spend a few minutes outside every morning. For example, you could move from Washington to Arizona. Or you could at least get a place with a balcony or a deck where you can conveniently sit outside every morning.

Some people like going for a morning walk, and if that’s your thing, perfect. Thing is I’m super lazy, and the whole idea of going outside and moving is anathema to me, so I like to cheat and just sit there reading for a bit.

Circadian Rhythm Disorders

Aha, finally we get to the promised Circadian Rhythm Disorders. They are divided into “intrinsic” ones (due to your own circadian rhythm) and “extrinsic” ones (due to external circumstances, e.g. shift work or jet lag).

Let’s here concern ourselves mostly with the intrinsic ones. You should never do shift work if you can avoid it, and we’ll talk about jet lag later on.

The intrinsic CRDs are as follows:

Advanced Sleep Phase Disorder (ASPD/ASPS, pathological morning lark)
Delayed Sleep Phase Disorder (DSPD/DSPS, pathological night owl)
Non-24-hour sleep–wake disorder (Non-24/N24, total failure to entrain to sunlight)
Irregular sleep–wake rhythm disorder (ISWRD, just a very weak rhythm that has you wake up and get tired irregularly)

(* These are also called Advanced Sleep Phase Syndrome/Delayed Sleep Phase Syndrome, so you’ll often see ASPS/DSPS instead of ASPD/DSPD. In fact, I know them as “syndromes.”)

The last one, ISWRD, was added relatively recently, after I started researching all this stuff. So I don’t know much about that one.

For reference, these distinctions were only added to the DSM-5 in 2013. So I find it hard to blame my childhood doctor for not recognizing the obvious Non-24 I’ve had since a very young age.

Examples

We’ve already met people with all of these disorders.

The girl I dated long ago could’ve probably gotten a diagnosis for ASPS. She didn’t seem aware of the condition, and I wasn’t aware of any of these at the time, or my own Non-24, so neither of us brought it up. Going to bed at 8pm is borderline a social disability. Her sleep phase is very “advanced” compared to most people’s, meaning she wakes & sleeps much earlier.
My friend who partied all night, and then hunkered down to work from 2am until after sunrise, only to sleep all day long, had DSPS. His sleep phase was much delayed from most people’s.
I have Non-24. Even with the most extreme sleep hygiene protocol, out camping, I won’t entrain to sunlight. Unless I’m doing keto, in which case I’m a regular old nightowl. In essence, unless I’m doing keto, I live in the cave experiment no matter where I actually am.

Orchestra Analogy again

Let’s torture the orchestra analogy some more, in case it helps, cause I understand these aren’t easy to grasp if you don’t have one yourself.

Someone with ASPS is always early on the rhythm, unable to await the correct timing from the conductor or the other musicians.
Someone with DSPS is always late, as if he’s seeing the conductor and hear the other musicians on a delayed video feed.
Someone with Non-24 is in a sound-proof room and can’t see the conductor at all; he might be able to follow a rhythm if he could hear or see one, but he can’t.

Non-24 Type I and Type II

This part is my personal observation, it’s not officially recognized medically. There seem to be 2 distinct types of Non-24. I call them Type I and Type II like with diabetes.

Type I: You have an extreme form of DSPS. Your rhythm might be so crazy long (30h?) that there’s no way in heck you can entrain it enough even to stabilize.
Type II: Your rhythm isn’t necessarily super long (mine is about 25h, only 30min off from the average), but the mechanism to entrain to sunlight is broken.

As an interesting aside, the vast majority of people with Non-24 are blind. It kind of makes sense that certain types of blindness would prevent you from receiving the entrainment signal from sunlight. Then again, some blind people can entrain - so it really depends on which parts of your eyes (or whatever is causing your blindness, e.g. nerves/brain) are affected.

(For the record, I am not blind. In case that wasn’t clear.)

Blindness is a good metaphor for Type II Non-24, I think. If I covered up your eyes or put you in the aforementioned cave experiment, and you had no way of getting any sunlight in your eyes, you’d be exhibiting Non-24 type symptoms.

Type II Non-24 is just that, except you’re not literally blindfolded/in a cave. But there’s something in the machinery of entrainment that isn’t working right.

This is the type that keto can potentially fix, as it did in me.

If you have Type I Non-24, and your circadian rhythm is impossibly long, keto won’t do anything for you.

Also if you’re blind, of course, keto won’t help you entrain. Keto only seems to fix an extremely specific niche problem that I and a few others have, whatever that problem may be.

Limited Capacity to Entrain

We’re nearly done exploring the biological fundamentals of our circadian rhythms. One important bit we’ll need is that we can’t just willy-nilly entrain to anything.

In fact, if we could, there would be no such thing as a circadian rhythm. If you could just step out of a cave, see the sun, and instantly be entrained, we wouldn’t need to talk about any of this. Conversely, if your rhythm instantly adjusted the second the sun set, you wouldn’t ever get jet lag, and my party/late work friend wouldn’t have had any issues.

Fundamentally, our circadian rhythm can only change so much per day. That used to be fine, because we couldn’t possibly outrun this capacity for most of human (or mammalian.. or .. life…) history.

Let’s look at an example, my party/late work friend with DSPS, and contrast him with 2 other people.

Imagine my friend’s circadian rhythm is about 27h, so he’s about 3h out of sync with the earth. Let’s call him Party Peter.
Now imagine the average person, with a circadian rhythm of 24.5h: only about 30 minutes out of sync with the earth. Average Abby.
Now imagine a person with a crazy long rhythm of 30h, “Long” John Silver. He’s not a pirate or anything, that’s just his name. He has a 6h difference to make up to stay in sync with the earth; pretty much impossible.

Average Abby only has 30 minutes to cover, she has no trouble entraining to the sun every day. As long as she doesn’t become a cave-dwelling programmer, and goes out at least a few minutes per day, she’ll be in a stable rhythm.

Party Peter has to overcome 3h. That’s significantly more difficult, and he can’t manage to compress his phase into a “normal” day like most people have. But he’s stable where he’s at, and doesn’t cycle around the clock.

“Long” John Silver can’t possibly entrain 6h of difference per day. He would have to fly from the East Coast (UTC-4) to Hawaii (UTC-10) EVERY SINGLE DAY just to stay “in rhythm” with the sun.

I haven’t seen any studies or experiments on people’s specific capacity to entrain. Anecdotally, it’s obvious that 24.5hh is nearly trivial to entrain to, since it’s the average person’s cycle length, and the average person has a somewhat stable rhythm.

Even 25h isn’t particularly difficult. When I’m on keto, all I need to do is get a few minutes of sunshine in the morning. When I don’t leave my house for 3 days in a row, I start noticing my sleep & wake times creep, first slowly, then faster. But all I have to do is force myself into the morning sun for 30 minutes for 2-3 days, and it’s back. But, the 1 hour difference - does make a difference compared to the average person. This will be important for Daylight Savings Time later on.

We really don’t know where the limit is, or where it starts to fall off. And is it individual? Maybe larks entrain extremely potently, whereas nightowls have diminished ability to entrain? Or maybe it’s just that their longer phases need more compression?

What we do know is that the ability to entrain is limited in everybody, which is why we have the phenomenon of jet lag.

Jet Lag

Ok, now we’re finally equipped to understand what jet lag even IS. Jet lag isn’t just that you’re tired & dehydrated after a long flight.

Jet Lag means that you ran away from your circadian rhythm, and it takes time for it to catch up to you, or “entrain” to your new location and sun exposure.

Jet lag is a modern phenomenon. It was only made possible with commercial jet aircraft, which allowed people to become severely out of sync with their circadian rhythms for the first time in history. Hence the name.

You can’t outwalk the sun, and you can barely even outdrive it: only if you were going full speed on a straight road for a whole day.

Remember, the continental U.S. has 4 time zones and hence a timezone differential of 3 hours. It is also about 2,000-3,000 miles to drive coast to coast.

If we use 3,000mi, you would need to average 125mph for 24h to cross it. That’s not particularly realistic unless you’re doing a cannonball run.

But if we allow for a few days, it’s pretty doable. I’ve driven nearly across in 3 days, although it was near the Southern border, where it’s closer to 2,000 than 3,000 miles. But it consisted of driving 10-16h each day, almost entirely on interstate highways I-10 and I-20 (shoutout to that nice truck that let me merge in front of him! 10-4).

Point being: even if you set out to out-drive your circadian rhythm, you’d have to go to extremes to do so.

I’m explaining this in so much detail because driving across the U.S. is a great metaphor for understanding the circadian rhythm and jet lag.

We just established that most people can relatively easily entrain by about 1h per day if they get maybe 30 minutes of morning sunlight. So if you only crossed one timezone in a day, like on my road trip, you wouldn’t even really get jet lag.

You’d drive 80mph for 8h a day, cover about 640 miles, maybe cross 1 timezone. But your capacity to entrain is also about 1h per day, so you wouldn’t even notice it much.

Example: 3h Jet Lag from New York to San Francisco

Let’s do an example and go through the numbers, and it should be very clear how to think about this.

One fine day you wake up at your normal, non-alarm wakeup time of 8am in New York City. You spontaneously decide to fly to San Francisco. You’ve always wanted to see the Golden Gate Bridge, and drive down that weird windy road. Besides, they don’t call it flyover states for nothing, right? Somebody’s gotta fly over them.

You take a jet from New York to SFO, covering a 3h timezone difference in half a day (incl. sitting in traffic on the 280 as your Uber driver keeps the windows open to save 0.2mpg vs. running the A/C).

As you arrive, it turns out you were racing your circadian rhythm. And since that guy hates flying, he drove his car. And when you touch down in SFO, he’s only made it to Columbus, OH. And that’s if traffic was great.

So the current situation is this: you are in San Francisco, and your circadian rhythm is in Columbus. It’ll take him another solid 2-3 days of driving to catch up to you.

You get sleepy and wake up according to your circadian rhythm, not your actual, geographic location on earth.

That means your first day in SF, you’ll wake up at your normal wakeup time - but in the Columbus timezone. Now Columbus, OH shares a time zone with New York, but make no mistake, they are over 500mi apart and the sun rises and sets 35 minutes apart, over half a time zone already. In other words, you’ll be getting up 3 hours earlier in terms of local time.

The second day, while you’ve been riding a bicycle over the Golden Gate Bridge and rode one of those cute cable cars, your circadian rhythm has been driving like a madman and has arrived in Omaha. That’s about 800 miles, not bad for a single day of driving. But the Midwest is empty and the speed limits are pretty reasonable, so it’s not out of the question. So on the third morning, you wake up on Omaha time.

On the third day, after you drive down Lombard St or saw the Painted Ladies or got mugged in a park or whatever it is people actually do in SF, your tireless circadian rhythm has made it to Salt Lake City. Another solid 950mi day in the books, you are now on Salt Lake time when you wake up on the 4th day.

Finally, as you anxiously wait for your circadian rhythm in your overpriced hotel room, because there isn’t anything more to do in San Francisco than 4 days’ worth, your circadian rhythm catches up to you on the fifth day.

He knocks on your door, exhausted from the long drive.

“Oh, you’re here,” you say. “Good, I’m just about to board the flight back home to New York.”

Your circadian rhythm would be in full rights to give you the finger and take a week off, but tireless as he is, he watches you get on the plane and then begins the long drive back home.

This is why the rule of thumb is that it takes about 1 day to recover from 1 hour of time zone difference/jet lag. Because 1h is roughly the capacity of the average person to entrain per day. Similar to how fast one could drive in a day, which makes it a great analogy.

Let’s write it up in a nice table:

Day    You   Circadian Rhythm                 Your local wakeup time
1      NYC   NYC (UTC-4)                      8am (still NYC!)
4      SFO   Columbus (UTC-4, >500mi away)    5am (8am Ohio time)
3      SFO   Omaha (UTC-5)                    6am (8am Omaha time)
2      SFO   SLC (UTC-6)                      7am (8am SLC time)
5      SFO   SFO (UTC-7)                      8am (8am SFO time)

It took your circadian rhythm 4 full days (from days 1-5) to catch up to your. It could be done in 3, or it could take longer. This depends on the person and what you do to entrain.

When you travel with a jet, your circadian rhythm lags behind you. Hence, “jet lag.”

All of this depends on many factors, e.g. you didn’t take a red eye and got super exhausted. You aren’t there for work, so you don’t have to wake up with an alarm clock. And so on. This is a very simple example to demonstrate how it works.

Cool, now we understand what jet lag is, and why you wake up super late when you fly east, and super early when you fly west (as in this case).

Direction-Based Entrainment Capacity.. Oh My

Ok, I lied to you. I said most people can easily entrain by 1h per day. That’s true… when going west, or waking up later (“delaying the phase”). It’s actually significantly more difficult for most people to entrain to the east, or earlier (“advancing the phase”).

How come?

Remember that most people’s circadian rhythm is longer than 24h, on average 24.5h long. That means that in order to “entrain” to a 30 minute time delay, the average Joe doesn’t even have to entrain. All he has to do is nothing. All he has to do is NOT compress his phase for one day. If he just stays indoors for 2 days, his rhythm will inherently have shifted later (“delayed phase”) and he’ll be right on time.

If you’ve done some coast to coast business travel, this will seem familiar. Living on the West Coast and flying to the East Coast for a few days is horrible. You suddenly have to wake up at what feels likes 4am to you every morning. You’re tired all day long, and collapse into bed when you get back to your hotel, exhausted. Yet you can’t even sleep! Your circadian rhythm is still on West Coast time, trying to stay up for another 3h.

But living on the East Coast and flying to the West Coast isn’t so bad. You wake up at 4am. You go to the hotel gym. Their breakfast is untouched when you show up at 6am, so you get your pick.

Yea, you get tired pretty early, and you go to bed at 9pm instead of midnight, or maybe 10pm if you stay for the social hour after a long day of meetings. But you sleep well, and wake up refreshed the next morning. After 3 days of meetings & cocktails on the company, you fly home and think to yourself: “That wasn’t so bad. I don’t get what these soft West Coast people are always whining about.”

Individual, Direction-Based Entrainment Capacity… FUUUU

Since we all have a different circadian rhythm, it’ll be different for everyone.

If you had a circadian rhythm of exactly 24h, you wouldn’t have to entrain to earth days at all. Even in a cave, you’d stay perfectly in sync with the sun.

Should you travel to the west by 1 timezone, you’d have exactly as easy a time to entrain as if you traveled to the east by 1 timezone.

But 24h is on the very short end of the bell curve; remember, the average person has a 24.5h circadian rhythm.

This means the average person has to delay by 30 minutes to accommodate a timezone shift 1h later (East to West, +1h - 0.5h), and advance/compress by 1.5h for a 1h timezone shift earlier (West to East, -1h - 0.5h). We’d thus expect the average person to take about 3x as long to advance vs. delay phase.

Let’s look at it on a 24-hour analog clock I drew expertly and beautifully for you, so it’s easier to visualize:

NYC → SFO

This is the example we did above, visualized.

The green “hands” represent the start & destination timezones, say, NYC to SFO in this case, represented by the green arrow. Note that we are flying WEST or LATER here.

To get back to 100% entrainment and synchronize our own sleep/wake time with the local clock time, our circadian rhythm has to either drive 3h later (“delay phase,” orange arrow) or a whopping 21h earlier (“advance phase,” blue arrow).

Naturally, pretty much everybody would choose the 3h drive. All you have to do as a New Yorker on the West Coast is stay up a little longer every day, and 3 days later, you’re entrained.

West Coast to Tokyo

Now let’s look at a more tricky example. Say we fly from the West Coast (UTC-7) to Tokyo (UTC+9), a whopping 16h timezone delay. Incidentally, this is the same as flying East for a 8h timezone advance, as the day wraps around at 24h. So this is the same situation, time zone wise, as flying from the West Coast (UTC-7) to Europe (UTC+1)

(If we were flying from the West Coast to Europe, imagine the green arrow going counter-clockwise for 8h instead of clockwise for 16h, ending up in the same spot.)

I consider this one of the most difficult jet lags out there, and that’s because we have a different capacity to entrain earlier vs. later. For me as a night owl, I can entrain roughly 2-3x as fast when trying to delay my sleep vs. advancing it. Simply staying up late and sleeping in comes natural, since my inherent rhythm is 25h anyway. Heck, all I have to do is not leave my house for a few days and I’ll automatically start delaying.

The reason this scenario is so tricky is that it’s not quite obvious which way to go. Since I can drive the longer, orange route at twice the speed of the shorter, blue route, I’m effectively equidistant from where I want to be.

In either case, I’ll likely take about 2 weeks to fully entrain, which is a very long time to be running on a different schedule.

Interestingly, this “most difficult” equidistant point probably depends on your own chronotype and cycle length. If your cycle was exactly 24h long, I’d expect the 12h timezone difference to be the most difficult for you, since you could “drive” the same speed in both directions.

How should YOU deal with jet lag?

In short, how you best deal with jet lag can depend a lot on your personal chronotype. You have to decide if you want to walk your internal clock earlier (“advance phase”) or later (“delay phase”). Which way is easier depends on the distance you have to cover, and the speed in which you can go in either direction, which is going to be biased towards “delay” for almost everyone, but to different degrees.

My intuition: the longer of a circadian rhythm you have, i.e. the more of a nightowl you are, the easier it’ll be to delay vs. advance an equidistant timezone difference. It would be cool to see more anecdotes or studies on people entraining to different timezone differences, so if you travel a lot or otherwise have experience with this, let me know how you adapt.

It can be important to know which strategy you’re choosing. If you’re intentionally delaying your phase, you should do “everything wrong” by normal standards. You should stay inside in the morning, delaying your first morning light as much as possible. You should watch movies on bright blue screens late at night. Remember, you WANT to delay your sleep!

On the other hand, if you’re trying to advance your phase, e.g. after a flight from the West Coast to the East Coast you should really, really go hard on the morning sunlight for a few days. And be really strict about evening lights and screens. It’ll just make your transition that much faster.

That said, there is a limit to how fast you can entrain, and all you can do is not stand in the way. The reality is that jet lag will occur if you travel long distances quickly, and you can’t make it go away.

Really messing up your Jet Lag entrainment

In certain situations it can be pretty easy to mess up your adaptation. I did this on a vacation to Europe a few years ago. As we just saw, this is a flight 8 timezones to the East, meaning I was now 8h earlier than my circadian rhythm, who was driving like a madman across the Atlantic.

I would wake up in the late afternoon or evening, while it was still daytime. This would send my body the signal to wake up earlier. Then I’d be awake all night and get tired in the early morning.

The problem was: the sun came up in the morning. Wanting to enjoy the sunrise, seeing things on my vacation, and general “morning sunlight is good for you!” thinking led me to take long walks in nature in the clock-morning, which was my circadian late-night. So I was also sending my body the signal to stay up and wake up later.

Of course this had the opposite effect I desired: now I was getting bright sunlight in the early morning AND late at night, sending mixed signals, arresting all adaptation.

I did this for a week until I realized I wasn’t entraining at all, but instead fighting my adaptation every day by getting conflicting sun input.

After giving up the morning AND evening sun exposure, I began to rapidly delay as is my nightowl nature, and soon I was waking up after sunset and then around midnight. From then on, it was all downhill, because morning sunlight now hit me at noon circadian time, which is fine and doesn’t send any wrong inputs.

In general, be wary if your post-flight, jet-lagged wake time includes the entirety of the night, and therefore bright sunlight at both ends. You might be sending mixed signals.

If so, make sure to choose which way around the clock you want to go, and that you’re avoiding going outside at a time that would prohibit you from entraining:

If trying to advance your phase (=wake up earlier), avoid sun before going to bed
If trying to delay your phase (=wake up later), avoid sun after getting up

Daylight Savings Time

Oh, my favorite! I hate Daylight Savings Time. Now that we know how the circadian rhythm and jet lag works, we are equipped to understand DST as well.

DST is only a 1h difference in timezones. In the spring, it goes 1h earlier (i.e. you are now 1h late) and in the fall, it’s the reverse.

That means the vast majority of people have a much harder time adapting to DST in the spring than going back in the fall. This is because of the direction-dependent capacity to entrain that we discussed above.

The big issue with DST is that it does NOT just take you one day to entrain.

What?! But it’s only 1h, you said we can easily entrain 1h per day!

Yes we can. But that’s only IF we entrain. We entrain to sunlight. You know what doesn’t change when DST changes? The sun.

That’s right, DST is like flying from SFO to SLC and incurring a 1h timezone penalty, but bringing your own sun with you! If you flew to SLC you’d ordinarily now have the ability to see the sunrise/sunset 1h earlier, allowing you to entrain within a day.

But DST doesn’t change the sun, only the clock time. So you cannot entrain! Or at least, you won’t automatically entrain simply by being in a new location like with actual jet lag. You have to make an active effort.

In essence, you have to act like you wanted to wake up 1h earlier than before DST: get more sun earlier, be careful of bright lights/screens more or earlier in the evenings.

Another way to say it: DST makes you incur 1h of jet lag every day, until the sun finally catches up to clock time. Depending on your location, that can take up to 2 months. Although, of course, it’ll gradually get better as sunrise moves by half a minute or so each day.

Modern Times: Everyone has slight DSPS

Man, fixing his circadian rhythm with a pair of spanners

This is the part most tips & tricks on sleep are trying to deal with. For example, avoiding screens and blue light, wearing blue light blocking glasses, and changing your computer screen to turn to a slightly orange/red hue in the evening.

Remember how I said you basically can’t entrain later via sunlight, unless you live in the arctic? That was true until electricity. While the sun will be down long before almost everyone is ready to go to bed, we can have our lights & screens on all night long.

Sitting in front of a bright TV or computer screen late at night is the equivalent of getting a little bit of sunlight exposure in the middle of the night. Naturally, this is very confusing for the circadian rhythm. Instead of compressing (“advance”) your phase, unlike morning sunlight exposure this will lengthen (“delay”) your circadian rhythm. That’s almost never what you want, unless you have ASPS like the girl I once dated, or you’re trying to overcome jet lag.

Since screens & artificial blue light were only invented a few hundreds years ago, and are now omnipresent in modern society, the effect is that everyone has symptoms of slight DSPS.

Yea, all those things might move the needle a little bit, depending on how much DSPS you’re currently giving yourself through late-night screen & light exposure.

I think the much bigger change is actually houses; and those are quite a bit older. If you spend your entire day indoors and never get any sunlight, you’ll have a much harder time entraining than Grok the Caveman, who had to leave his cave to take a leak first thing in the morning, and then probably spent most of his day running around hunting mammoths and climbing trees & picking fights with honey bees. It’s unlikely that many pre-industrial people would ever have failed to entrain to the sunlight, since they would’ve spent the majority of their time outdoors.

The blue light thing is true, blue frequencies are the part that hits the button the most. That’s why you can fall asleep staring into a campfire.

Personally, I don’t wear blue blockers. I do have set my computer to turn orange at night, like many people.

But I think that just getting some morning sunlight is by far the biggest lever. I can fall asleep while watching an action movie if I’m well-entrained. Close the laptop, head on the pillow, and I’m gone 3 seconds later.

One aside: I notice that when I’m very well entrained, my sleep isn’t just earlier in my personal genetic nightowl spectrum, it’s also more compressed and my sleep quality is much better. I fall asleep instantly, and I wake up, jumping out of bed ready to tackle the day. I’m more rested while sleeping less.

When I’m not entrained, the change from wakefulness to tired and vice versa takes hours, and I lay in bed exhausted, unable to fall asleep, and wake up dazed & confused and wanting to snooze. I usually notice this within 3 days of not getting ANY sunlight. (Yes, this happens to me. I’m a programmer.)

What can you do with all this information?

A lot of this was very theoretical, but I also want to leave you with some practical tips on how to utilize this knowledge.

Know your Chronotype

Know Thyself is the main part. If you’re a natural night owl, you will never be a successful, happy morning lark. Don’t fool yourself, it’s genetics. That doesn’t mean you can’t improve your sleep, it just means be realistic and accept the limits of biology.

Most people are kind of DSPS’d from screens & lights & lack of morning sunlight, so they can easily move their clock earlier by 30 to 60 minutes just by practicing some very light light hygiene (hah). Configure your computer screen to go orange at night, turn the brightness down as much as you can at night, and drink your coffee on the porch while checking emails in the morning. That’s an easy win and will probably buy you quite a bit.

Beyond that, you might have to get pretty extreme. Most people will have a hard time getting enough sunlight in & avoiding artificial lights enough to advance phase 2h or more. It’s not impossible, but it’s probably at the limit of what can be reasonably done.

How do you even know your real Chronotype?

Most people have never not had social obligations and some haven’t woken up without an alarm clock in decades, or even their entire lives. School, parents, then work, maybe even the military, then you’re a parent.. how do you even know your chronotype if you’ve always woken up with an alarm?

The answer is: free-running sleep. Free-running sleep just means that you don’t use an alarm, and you minimize all other sources of external waking calls. No spouse or kids waking you up in the morning. No phone calls. Let your body wake up when it’s ready to, and only go to bed when you’re tired enough to actually fall asleep.

You don’t need to do this forever, but you do need to do it for a certain amount of time to see a steady pattern.

I’d say 7 days is about the minimum. After 30 days you’d definitely know, but most people won’t need nearly as much.

A weekend is not enough, because you might just still be a little bit sleep deprived from waking up with an alarm for work, and make up for it by sleeping in. But that should settle after 3-4 days.

So when you next plan a vacation, consider staying at home, telling your spouse not to wake you in the morning, and write down your sleep & wake times. Or you can use one of a myriad of phone apps that record it for you.

If you’re used to waking up with an alarm, your natural wake time will likely be a bit later. (Or you wouldn’t need the alarm.)

Let’s look at an example. You take a week off and decide to just stay at home, sleep in the guest bedroom for the week, and tell your family not to wake you up in the morning.

(including 2 work days so we can see your wake time with alarm)

Day    Wake time         Comment
Thu    6am               Still working
Fri    6am               Still working
Sat    11am              Sleep in to make up for work
Sun    10:30am
Mon    10am              Start of vacation
Tue    10am
Wed    9:30am
Thu    9am
Fri    9am
Sat    9am
Sun    9am

The first weekend you’re still tired from work, when you had to wake up with an alarm clock. But that doesn’t last long and soon your wake up time settles around 9am. You naturally start getting tired around 11:30pm, and tend to go to bed at almost 1am. That puts your mid sleep point at 5am. Let’s consult our chart from above, which I’ve copy & pasted so you don’t have to scroll a mile.

Aha! Mid sleep of 5am puts you in the right third of the normal distribution, you’re definitely at least a light night owl. Probably not severe, but you’re not a real morning person either. If you were to really work on it, get that early morning sunlight, be super diligent about lights & screens at night, you might be able to shift your free running sleep to 8am, maaaaybe even 7:30am. But it’s highly unlikely you’ll ever get to 6am without an alarm. That would be a 3h advance, pretty dramatic.

See, it wasn’t so hard - all you had to do was take 7 days off and free-run, vacationing in the comfort of your own home. Heck, I bet you even mowed the lawn! Now you know your rough chronotype and can go from there.

Practice some easy sleep hygiene & do the low-hanging fruit stuff I mentioned above, and you’ll have a much easier time waking up in the morning when you go back to your alarm.

Design a System for Morning Sunlight

I’m avoiding the word “habit” here. Don’t try to force yourself to get into the sun. Make it easy for yourself to get into the sun first thing in the morning. That way, you don’t need to use any willpower.

My system: I like reading, I like chatting with friends, and I like drinking coffee. All of these can be done outside. So I put a little camping chair & table outside and take my phone or a book outside with my morning coffee. Then I can happily reply to messages, check the news, read my book, and I just happen to be doing it in direct sunlight.

No willpower needed.

DST or jet lag? Pick a strategy

When DST rolls around or you go on a longer trip, maybe abroad, you’ll know what will happen to you and why. You can make an informed decision on how to deal with it. If the trip is so short that you won’t have much time left after you fully entrain to the new location, don’t bother (e.g. a 7 day trip to Tokyo). Your sleep’s gonna be all over the place and you’ll have to go back before you’re done entraining.

But if you’re going for a month, you can pick an optimal strategy. It can make a huge difference if you only spend 7 days fighting your sleep, and can enjoy 3 weeks of your vacation, or if you spend the entire time fighting your own biological clock.

Low-Dose Melatonin as an Intervention

Lastly, maybe the most acute tip for things like jet lag or DST or other short-term interventions: you can keep around a bottle of low-dose (e.g. 0.5mg) melatonin.

The typical recommended usage is to take it about 2h before your desired sleep time. It’s like lighting a fire under the smoke detector, basically kickstarting the machinery that will make you tired and ready for bed.

It takes some time for this machinery to spin up. I know of some people with heavy DSPS who’ve used melatonin daily for years or even decades, basically dragging their circadian rhythm earlier kicking & scream every day. Some of them even take the melatonin as early as the early afternoon, to counteract their extreme cycle lengths.

If you have pretty normal sleep and don’t regularly stay up until 4am unable to sleep, you are probably fine just doing 2h before sleep.

After DST changed this year, I just took 1 of the 0.5mg melatonin tablets around 10pm, and I’d feel myself get pretty tired around midnight. I did this for about a week, and then didn’t feel the need any more and discontinued it. It was by far the easiest and least-painful DST transition in recent memory.

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