Jet Lag Planner

Your Body Clock: The Circadian Rhythm That Runs Your Jet Lag

The first time jet lag truly floors you, you lie wide awake at 3 a.m. in a hotel bed feeling completely alert, then spend the following day face-down useless with your head propped on a conference table by noon. What's easy to miss in that state is that a pinhead-sized clump of brain cells behind your eyes is keeping its own private time, stubbornly, and everything you feel that day is that little clock arguing with the one on the wall. That clump is your body clock, or circadian rhythm: a roughly 24-hour timer that decides when you feel sleepy, alert, hungry and warm. Jet lag is simply what happens when you cross time zones faster than the clock can follow, so your inside time and the local time stop agreeing. Learn one landmark on that clock, the moment your body runs coldest in the small hours, and the whole business of timing light suddenly starts to make sense.

What a circadian rhythm actually is

"Circadian" comes from the Latin circa diem, "about a day," and that vagueness is half the point. Almost every tissue you own runs on its own near-24-hour cycle: sleep and waking, core temperature, the hormones melatonin and cortisol, alertness, even digestion. These aren't a drawer full of separate timers each ticking away to itself. One conductor at the top keeps the whole orchestra in step, and that conductor is deaf to the outside world unless something resets it every single day, and that something is light.

The master clock behind your eyes

The conductor has a name: the suprachiasmatic nucleus, a paired cluster of roughly 20,000 neurons, parked in the hypothalamus directly above the spot where your optic nerves cross. It gets its light signal piped straight down a private line from your retina, then keeps the smaller clocks scattered through your organs marching in time with each other and with the day outside.

That direct wiring to the eye is the reason light beats every other trick for moving the clock, which is the whole argument of Light: The Most Powerful Jet-Lag Lever. Brightness gets measured in units called lux, and the gap between indoors and out is enormous: a well-lit office manages a few hundred lux, a dull overcast sky still throws around a thousand, open daylight runs 10,000 to 25,000, and direct sun tops 100,000. To a clock built to read the open sky, a day spent under office lighting barely registers as daytime at all.

Why your clock runs slow

One quirk explains a surprising amount of your jet-lag misery. Sealed away under careful dim lighting with no clocks and no daylight, the average human clock ticks over roughly every 24.2 hours, not a clean 24. That spare slice of an hour sounds trivial. Because the clock runs a little long, though, it drifts later every day unless something reels it back in, and morning light is the thing that does the reeling.

That built-in pull toward later is why staying up an extra hour feels effortless while dragging yourself to bed early feels like a small war with yourself. It's the same quirk that makes flying east hurt more than flying west, which is the subject of Why Flying East Is Harder Than Flying West.

Your temperature low point: the hinge for light

Your core temperature isn't flat across the day. It rises and falls by about half a degree Celsius and bottoms out in the last hours of sleep, and that low point is the single most useful landmark on the whole clock. Its scientific name is a mouthful, the core body temperature minimum, or CBTmin for short, and it's the hinge the entire light plan swings on. The rule around it is short enough to keep in your head: bright light in the hours before your low point pushes your clock later, and bright light in the hours after it pulls your clock earlier.

Finding yours needs no equipment, only a little arithmetic. Count back a few hours from the time you normally wake: the clinical rule of thumb is three hours, and the CDC gives a wider window of two to four. If you're usually up at 7 a.m., your low point sits somewhere around 3 to 5 a.m., call it 4. Catch bright light in the hours just after that and you shift earlier; catch it in the hours before it and you shift later; muddle the two and you'll move the wrong way. That's exactly how a well-meaning traveller ends up making their own jet lag worse. The protocols use a four-hour window either side, which is generous enough that you don't need to pin the minute down. Night owls and early larks will find these numbers slide around a bit, a story Larks and Owls: How Your Chronotype Changes the Plan tells properly.

What jet lag is, in clock terms

This is the exact moment jet lag is born. Board a flight across a stack of time zones and your suprachiasmatic nucleus stubbornly stays on home time for days. You land with your inner clock and the local clock badly out of register, so the daylight now hitting your eyes turns up at what your body swears blind is the wrong hour. Out come the familiar symptoms: sleepiness at ridiculous times, the 3 a.m. wake-up, a fog that won't lift all afternoon, a stomach with no idea when lunch is meant to happen.

That is a different animal from plain travel tiredness, and people muddle the two constantly. Travel fatigue is straightforward exhaustion from a cramped, dehydrating, sleepless journey, and one solid night's rest mostly sorts it out. Jet lag is the stubborn one: your body clock out of step with local time, dragging on for days because the clock can only move so fast. Left to its own devices it re-syncs at an average of about 1.5 hours a day heading west and 1 hour a day heading east, and aiming your light around that low point is how you hurry it along, to roughly 2 hours and 1.5 hours a day respectively. If you want the fuller picture of the symptoms and the timeline, What Is Jet Lag? Causes, Symptoms, and How Long It Lasts lays it all out.

Fun fact

In 1729 a French astronomer named Jean-Jacques d'Ortous de Mairan noticed his mimosa plant opening its leaves every morning and folding them away every night. Ordinary enough, until he shut it in constant darkness to see what would happen. The leaves carried on opening and closing right on schedule, with no daylight anywhere to cue them. It was the first real hint that living things carry a clock inside rather than simply reacting to the sun. Yours is doing precisely the same thing this very second, even while you're sealed inside a windowless plane.

How the Jet Lag Planner uses your low point

The Jet Lag Planner works out roughly where your temperature low point falls from the sleep and wake times you type in, then pins each day's "seek light" and "avoid light" windows to it, so you're always nudging the clock the direction you actually mean to go. It can't take your temperature, obviously, so it reads your schedule alongside the size and direction of your trip and gives you a strong starting point rather than a lab measurement. Your own timing will wander a little from what it predicts, and that's completely normal.

Sources

General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition.

Last reviewed August 5, 2026.

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Jet Lag Planner

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General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition. © 2026 XOR Limited.