How to Beat Jet Lag: What the Research Shows

• Travel fatigue ≠ jet lag. The first resolves with sleep. The second is a clock offset and persists for days.
• Eastward is harder because advancing the clock fights its natural drift. West delays; delay is easier.
• Light is the strongest lever — and its effect reverses direction depending on when you get it.
• The pivot point is your core body temperature minimum, roughly 2–3 hours before your usual wake time.
• Melatonin works for crossings of 5+ zones. Low doses (~0.5 mg) match high ones; timing is the active ingredient.

What jet lag actually is

Deep in the hypothalamus sits the suprachiasmatic nucleus, a cluster of around 20,000 neurons that acts as your master clock. It governs far more than sleepiness: core body temperature, cortisol release, melatonin secretion, digestive enzyme production, alertness and reaction time all run on schedules it sets.

That clock is not driven by the sun directly. It runs on its own, and in most people its natural period is slightly longer than 24 hours — around 24.2. It is kept in step with the outside world by external cues, of which light is overwhelmingly the most powerful.

Fly across six time zones and every system it governs is now six hours out of phase with your surroundings. Your body produces cortisol to wake you at 3am local, drops your core temperature for sleep in the middle of the afternoon, and releases digestive enzymes when nothing is being served. The symptoms — night waking, daytime fog, gut upset, mood swings — are not a single malfunction. They are dozens of correctly functioning systems all running on the wrong timetable.

This is why "just sleep more" fails. You are not short of sleep. You are misaligned.

It is also worth separating out travel fatigue: the dehydration, stiffness, noise and disrupted meals of the journey itself. That is real, and it resolves after one good night. If you still feel wrong on day three, that is jet lag.

Why east is worse

Because your internal period runs slightly long, your clock naturally wants to drift later each day. Left in constant dim light, most people go to bed a little later every day.

That asymmetry decides everything:

• Flying west (London to New York) means your clock must delay. You are asking it to do what it already wants to do. Most people can delay by 1–2 hours per day.
• Flying east (New York to London) means your clock must advance. You are working against the drift. Most people manage about 1 hour per day, sometimes less.

So six zones west is roughly a three-to-four-day adjustment; six zones east is closer to six. This is the physiological reason behind the well-worn observation that the trip out is fine and the trip home is brutal, or vice versa depending on your direction.

For very large eastward shifts — around ten zones or more — the advance is so hard that the body sometimes adjusts by going the other way around the clock, delaying by fourteen hours instead of advancing by ten. Some travel-medicine guidance suggests deliberately treating very large eastward shifts as westward ones for this reason.

Light: the strongest lever, and the easiest to get backwards

Light is the dominant signal your clock listens to. But — and this is the part almost every travel article omits — its effect depends entirely on when you receive it, and it reverses.

The relationship is described by a phase response curve, and its pivot point is your core body temperature minimum (CBTmin): the coldest point of your daily temperature cycle, which occurs roughly two to three hours before your habitual wake time. If you normally wake at 7am, your CBTmin is around 4–5am.

From that pivot:

• Light in the hours BEFORE CBTmin → delays your clock (pushes you later).
• Light in the hours AFTER CBTmin → advances your clock (pulls you earlier).

Now the trap. Say you fly from New York to London — six zones east, so you need to advance. You land at 7am London time, walk out into bright morning sun, and feel virtuous.

But 7am London is 1am New York, and your CBTmin is still on New York time at around 4–5am NY, which is 10–11am London. That bright morning light arrives before your CBTmin — so it delays your clock. You have just pushed yourself further from London time while doing what every guide told you to do.

The correct move on that first morning is to avoid bright light until mid-morning — sunglasses outdoors, dim indoors — then get strong light exposure from late morning onward, which now lands after your CBTmin and advances you. As your clock shifts earlier each day, your CBTmin moves earlier too, so you can start seeking light progressively earlier.

Westward is more forgiving because you need a delay and evening light is easy to arrange:

Direction	You need	Seek light	Avoid light
Eastward	Advance	Late morning to afternoon, moving earlier each day	Early morning on arrival day
Westward	Delay	Late afternoon and evening	Early morning

Bright outdoor daylight is far stronger than any indoor lighting — a cloudy day outdoors typically delivers many times more light than a well-lit office. Getting outside beats any device.

Melatonin: what the evidence supports

Melatonin is the hormone your pineal gland releases as darkness falls; it is the clock's chemical announcement that night has begun. Taken as a supplement, it acts as a timing signal rather than as a sedative.

A Cochrane systematic review of randomised trials concluded that melatonin taken close to target bedtime at the destination was effective at reducing jet lag from flights crossing five or more time zones, with the effect most pronounced for eastward travel. Subsequent reviews have found that 0.5 mg is about as effective as 5 mg for phase shifting — which is a genuinely useful finding, because typical over-the-counter doses are three to ten milligrams, far more than the evidence requires and more likely to leave you groggy.

Three practical points:

Timing is the active ingredient. Melatonin's own phase response curve is roughly the inverse of light's: taken in the local evening it advances the clock; taken in the local morning it delays it. Taken at the wrong point it makes things worse, not just less good.

Take it close to target bedtime at the destination, starting on the day of arrival and continuing for a few nights, rather than trying to reason about your CBTmin.

Supplement regulation varies widely. In the US melatonin is sold as a dietary supplement with correspondingly loose oversight; in the UK, EU, Australia and elsewhere it is a prescription medicine. Product content has been found to vary substantially from the label in some markets. If you take other medication, are pregnant, or have epilepsy or an autoimmune condition, check with a pharmacist or doctor first.

A protocol that works

Before you fly (optional, and it helps). For two or three nights before an eastward trip, shift your bedtime and wake time one hour earlier per night, and get bright light on waking. For westward travel, shift one hour later. This is unglamorous and effective — you arrive with part of the adjustment already done.

On the plane. Set your watch and phone to destination time as you board and start thinking in it immediately. Sleep if it is night at your destination and stay awake if it is day, using an eye mask and earplugs to enforce the choice. Skip alcohol — it fragments sleep architecture and worsens the dehydration of a dry cabin. Drink water. Move around every couple of hours, mostly for circulation rather than circadian benefit.

On arrival. Get onto local meal and sleep times immediately, even if you feel wrong. Do not go to bed at 4pm because you are shattered; push to at least 9pm local. Apply the light schedule from the table above. If you must nap, keep it to 20–30 minutes and before mid-afternoon — a two-hour nap will wreck the following night.

Caffeine, used deliberately. It genuinely improves daytime alertness during adjustment. It also has a half-life of around five to six hours, so a 4pm coffee is still working at 10pm and will delay a clock you are trying to advance. Morning and early afternoon only, especially heading east.

Meals as a secondary cue. Meal timing does exert some influence on peripheral clocks in the liver and gut, and eating on local schedule is easy to do. Treat it as a helpful supporting signal rather than a lever — the evidence for aggressive fasting protocols is much weaker than their popularity suggests.

Short trips: do not adjust. Under about three days, staying on home time is usually the better play. You would barely finish adjusting before flying home and then face a second adjustment. Keep important commitments inside your home-time daytime window where the schedule allows.

What does not work

Hydration. Worth doing for comfort. Does nothing to shift a body clock.

"Powering through" day one. Staying up 30 hours does not reset anything. It adds sleep deprivation on top of misalignment, and the two are separate problems.

Sleeping pills as a fix. They can help you sleep on a specific night, at the cost of grogginess, and they do not shift the underlying clock. They treat a symptom.

Aggressive multi-day fasting protocols. Popular, widely repeated, and supported by much thinner evidence than the confidence with which they are recommended.

The honest summary: you cannot eliminate jet lag, because a body clock physically takes days to move. What you can do is stop accidentally pushing it the wrong way — and on that measure, understanding when not to seek light is worth more than any supplement.

Further reading: The CDC's Yellow Book chapter on jet lag is the most reliable free clinical reference. The melatonin evidence is summarised in the Cochrane review Melatonin for the prevention and treatment of jet lag. Helen Burgess's practical guide to using bright light and melatonin sets out the timing logic in detail.

This article is general information, not medical advice. Check with a clinician before using melatonin if you are pregnant, taking other medication, or managing a health condition.