Jet lag: circadian misalignment and how to beat it

Jet lag isn't tiredness in the ordinary sense, and it isn't cured by a long sleep the way ordinary tiredness is. It's a mismatch between two clocks: the one on the wall at your destination, and the one running inside your own brain. A flight can move your body across half the planet in a matter of hours, but the internal clock that governs when you feel sleepy, alert, hungry and sluggish doesn't move that fast. It has to be dragged, hour by hour, to the new time zone — and until it catches up, you feel it.

A clock inside your head

Almost every process in the human body runs on a roughly 24-hour cycle, coordinated by a master pacemaker in the brain called the suprachiasmatic nucleus, a small cluster of cells sitting in the hypothalamus just above where the optic nerves cross. This pacemaker doesn't just control the sleep-wake switch. It also drives the daily rise and fall of core body temperature and times the release of hormones such as melatonin, which climbs in the evening to promote sleep, and cortisol, which rises before waking to promote alertness. Left with no external cues at all, most people's internal clock does not settle on exactly 24 hours — it tends to run a little longer, which turns out to matter a great deal for how jet lag behaves.

Normally this clock is kept in step with the actual day by external time cues, the strongest of which is light hitting the eye. Wake up, see daylight, and the clock is gently reset each morning before it can drift away from 24 hours on its own. Fly across several zones, though, and that daily reset suddenly points at the wrong time of day entirely. Your clock is still telling your organs it's mid-afternoon in the place you left, while the sun outside is setting, or rising, on an entirely different schedule.

Why the symptoms feel so scattered

The fatigue, the insomnia at 3am, the fogginess, the upset stomach — jet lag's symptom list looks less like one problem than several, and in a sense it is. Different organs and systems realign to a new time zone at different speeds: core body temperature and the sleep-wake cycle typically shift faster than digestion, and hormone rhythms can lag behind both. Researchers call this "internal desynchrony" — for a few days, parts of your body are effectively living in different time zones from each other, not just from the clock on the wall. That's a large part of why jet lag feels like more than simple exhaustion: your gut may still be running on departure-city time while your eyes are struggling to stay open in destination-city daylight.

Why east is harder than west

Anyone who has done both knows a flight that crosses time zones eastward — Europe to Asia, or North America to Europe — tends to hit harder than the same number of zones travelled west. The reason traces straight back to that internal clock running slightly longer than 24 hours. Travelling west lengthens your day, asking the clock to delay, which lines up with its natural tendency and feels closer to simply staying up late. Travelling east shortens your day, asking the clock to advance — effectively to go to bed and wake up earlier than it wants to — which fights against the clock's own momentum. A rough and commonly cited rule of thumb is about one day of adjustment for every time zone crossed, though eastward trips often sit at the slower end of that range. Our jet lag guides by route break this down for specific city pairs, including how many zones and which direction each particular trip crosses; the time difference calculator is the quickest way to check exactly how many hours any given trip involves before you fly.

Light is the lever that moves the clock

Because the suprachiasmatic nucleus takes its main cue from light reaching the eye, light exposure is the single most powerful tool for shifting it — and badly timed light can push it the wrong way just as easily as well-timed light helps. After travelling east, the aim is to advance the clock, so seeking bright light in the morning at your destination and avoiding bright light late in the evening tends to help pull the clock forward. After travelling west, the aim is the opposite: seeking evening light and avoiding early-morning light helps delay the clock to match the new, later local time. Around a large time shift, the body's sense of when "morning" and "evening" actually are can itself be confused for the first day or two, which is why light exposure right around subjective dawn and dusk needs particular care — light at the wrong moment can nudge the clock backward instead of forward.

A realistic toolkit

None of the following eliminates jet lag; they reduce its depth and shorten how long it lasts, which for most travellers is the realistic goal. A few days before departure, gradually shifting bedtime and mealtimes an hour or so toward the destination's schedule gives the clock a head start. On arrival, switching immediately to local meal times and local sleep times — rather than eating and sleeping on departure-city time — gives the body consistent cues to work from instead of mixed signals. Correctly timed, low-dose melatonin is one of the better-studied aids: taken close to destination bedtime, it can reinforce the "night" signal your clock is trying to catch up to, though timing relative to the direction of travel matters and it's worth checking with a pharmacist or doctor, particularly if you take other medication. Short naps can relieve acute sleepiness but are worth keeping brief and early in the day, since a long or late nap can undo the night's progress toward the new schedule. Caffeine timed to local daytime, staying hydrated through a dry cabin, and going easy on alcohol — which fragments sleep even when it helps you doze off initially — round out a sensible, unglamorous approach that genuinely helps most travellers arrive in better shape.