Midnight in your body

How your brain keeps time, what sleep loss and jet lag actually do to it, and how to shift your clock on purpose, with a planner that builds a day-by-day plan for your own trip.

By Reid Robison, MD. 3,360 words, with an animated body clock, a sleep-pressure model, the light timing curve, a sleep-loss timeline, an east-versus-west chart and a jet lag planner you can save, print or send.

Researched and reviewed by Reid Robison, MD × OpenEvidence × Claude How this post was made

This question has come up a lot lately in my clinic and among friends, so I made a two-page handout for patients flying from Utah to Europe. This post is the bigger version: the brain science behind it, what the evidence says about each tool, and a planner that works for any trip, not just that one.

1The clock in your brain

Deep in your brain, just above where the optic nerves cross, sits a cluster of about 20,000 neurons called the suprachiasmatic nucleus. It's your master clock. It times when you feel sleepy and alert, when your body temperature dips and rises, when hormones like melatonin and cortisol are released, and even when your gut expects food. Nearly every organ has its own clock, and the master clock keeps them in step.²⁹

The clock runs on its own, slightly longer than 24 hours in most people, and it's reset every day mostly by one signal: light reaching special cells in your eyes. Fly across time zones and the light-dark cycle outside jumps instantly. The clock inside can't. It shifts only about an hour a day after flying east, and a bit faster after flying west.¹ Play the trip below.

2Two forces: pressure and timing

How sleepy you feel at any moment comes down to two forces. The first is sleep pressure: the longer you're awake, the more a sleep-promoting signal (adenosine is a big part of it) builds up, and sleep clears it. The second is your body clock, which pushes for alertness through the day and opens the gate for sleep at night, releasing melatonin, the "darkness hormone," about two hours before your usual bedtime. This is often called the two-process model of sleep.¹⁰¹¹

At home the two forces line up: pressure is highest just as the clock opens the sleep gate. After a long flight they don't. Toggle between home and the first day in Europe.

3Light: the reset button

Here's the single most useful idea in this post. Light doesn't just "wake you up"; it moves your clock, and the direction depends on when in your body's day it hits. Light in the hours before your body's temperature low, which usually falls 2 to 4 hours before your normal wake-up time, pushes the clock later. Light in the hours after that low pushes it earlier.¹²⁴ This is called a phase response curve.

That's why the morning sun can betray you. Land in Paris at 8 a.m. after flying 8 hours east, and your body's temperature low, around 5 a.m. Utah time, is still ahead of you at about 1 p.m. Paris time. Bright sun at 9 a.m. lands before the low, and pushes your clock later, the wrong direction. The fix sounds backward: sunglasses and indoor light in the early morning for the first day or two, then outdoor light from about midday.³⁴

Melatonin works almost like the mirror image. Taken in your body's afternoon or early evening, it nudges the clock earlier; taken in your body's morning, later.¹³ For an eastward trip, that makes your new local bedtime a good time to take it.

4What sleep loss does, hour by hour

Jet lag usually arrives with a second problem: a short or broken night on the plane. Sleep loss is its own impairment, separate from the clock. A classic study found that after 17 hours awake, performance on a tracking task was about as impaired as with a blood alcohol of 0.05 percent, and after 24 hours, about 0.10 percent, above the US legal limit for driving.⁶ A later study in professional drivers put the 0.05 percent mark at about 17 to 19 hours awake.⁵ Slide through a long travel day.

Short sleep also dulls attention, mood, memory and judgment, and it makes jet lag itself worse.¹⁴¹¹ That's why the common advice to stay up late before a flight "so you'll sleep on the plane" backfires. The better move is to bank sleep: eight to nine hours a night for three to five nights before you leave.¹⁵⁴

5Why east is harder than west

Most people's internal day runs slightly longer than 24 hours, so staying up later comes more naturally than going to bed earlier. Flying west asks your clock to run later, which it does readily; flying east asks it to run earlier, which it resists. In studies of real travelers, clocks shifted about 57 minutes a day after eastward flights and about 92 minutes a day after westward ones.¹ Sports medicine guidance rounds these to about 1 hour a day east and up to 2 hours a day west.¹⁵

For very large eastward jumps, around 8 to 10 hours or more, it can be easier to let your clock go the long way around, shifting later instead of earlier. The planner below switches strategies at 10 hours east; at 8 or 9, most people do fine shifting earlier as long as they avoid early-morning light at first.¹¹⁵ And for short trips of two or three days, staying on home time is often the smarter choice.⁴

6The tools, and what to skip

Light and darkness, at the right time

The most powerful tool. Seek light in the 0 to 3 hours after your body's temperature low when you've flown east, and in the 0 to 3 hours before it when you've flown west; avoid it in the opposite window. Outdoor light for 30 to 180 minutes works, and sunglasses or indoor routes handle the avoid windows.⁴³ The American Academy of Sleep Medicine also suggests, as an option, moving your sleep schedule an hour earlier a day for three days before flying east, with bright light in the morning.⁷

Melatonin, at the right time

A timing signal for your clock, not just a sleeping pill. For eastward trips, take 0.5 to 5 mg of immediate-release melatonin at your new local bedtime for about four or five nights, and on the plane as you settle down to sleep in destination nighttime. No dose is proven best, and low doses still shift the clock.⁷⁸ In a Cochrane review, melatonin reduced jet lag in 9 of 10 trials when taken close to the target bedtime at the destination.¹⁶ In the US it's a supplement, so choose a third-party verified brand.

Protect your sleep, and plan when you're awake

Bank sleep before you go. On an overnight eastbound flight, set your watch to destination time, eat early, then sleep with an eye mask and earplugs, and go easy on alcohol, which fragments sleep. On arrival day, stay up until a normal local bedtime, with one 20- to 30-minute nap in the early afternoon if you need it.¹¹¹⁵

Caffeine, with rules

Useful for daytime sleepiness, in 100 to 200 mg doses. But caffeine late in the day disturbs sleep and can push your clock later: in a controlled study, a double espresso's worth taken 3 hours before bed delayed the clock by about 40 minutes.¹⁷ Stop 8 to 10 hours before bed.⁴

Prescription sleep medicine

Better evidence than over-the-counter sleep aids: zolpidem 10 mg improved sleep for three to four nights after flying east across five to nine time zones. It doesn't reset your clock, it can cause confusion or memory gaps, and deep sleep on a plane may raise clot risk. Test a dose at home first, and ask your clinician.³⁷

7Plan your trip

Enter your flight and your usual sleep times, and the planner builds a day-by-day schedule: when to sleep, when to seek and avoid light, when caffeine is fine, and when to take melatonin if you use it. It estimates your body's temperature low at about 2.5 hours before your usual wake time and moves it about an hour a day after flying east and 1.5 hours a day after flying west, using the timing rules from the sports medicine consensus.¹⁵⁴ It starts empty; load the example to see how it works.

A few honest caveats. The timing rules come mostly from laboratory studies; in real travel, daylight, schedules and individual clocks vary a lot.⁴ Night owls and early birds will differ from the estimate. Treat the plan as a starting point, not a prescription. Apps such as Timeshifter build similar plans, and the CDC and sports medicine reviews mention them.¹¹

8What about creatine?

I get asked about this more and more. The short answer: no study has tested creatine for jet lag, and it doesn't shift the body clock. The interest comes from studies of acute sleep deprivation, where it modestly reduced some of the cognitive decline from sleep loss.²²

• A single large dose. In a randomized, double-blind crossover trial during a night of partial sleep deprivation, 0.35 g/kg (about 25 g for a 70 kg adult) raised brain creatine on MR spectroscopy, prevented a drop in brain pH, and improved processing speed, short-term memory and reaction time, with less fatigue than placebo. Effects began about 3.5 hours after the dose, peaked around 4 hours and lasted up to about 9.²³
• Loading doses. At 20 g a day for 7 days, creatine improved choice reaction time and some executive tasks over 24 hours awake and lessened fatigue, without changing verbal or spatial recall.²²
• The overall picture. Reviews find the most consistent gains in processing speed, with mixed or null results for memory and attention, and little benefit in rested, healthy young adults. Older adults, vegetarians and people under stress such as sleep loss may benefit a bit more.²²²⁴²⁵

So at most it's an optional add-on for the sleep-loss part of the first day or two, not for the clock part, and never a substitute for timed light, melatonin and protected sleep. If you try it, keep it to the morning: in animals creatine lowered sleep pressure, and whether a late dose affects human sleep is unknown.²⁵ The 0.35 g/kg dose is far higher than the usual 3 to 5 g a day, and its safety hasn't been established in older adults or people with kidney disease.

How this post was made

I use AI tools in building this site, and I'd rather be open about how. For this post, I set the scope and the clinical judgment and wrote the patient handout it grew from. Claude helped draft the text and build the interactive tools. The key clinical claims, including the jet lag recommendations and the creatine and kidney safety sections, were checked against OpenEvidence, an evidence search tool built for clinicians that cites the medical literature. Then I reviewed every number and every recommendation against the sources listed below. Any errors are mine.

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