For jet lag, the evidence on melatonin is unusually strong. Timing is the catch.

Of all the supplements marketed to travellers, melatonin is the rare one with genuinely strong evidence behind it — but only for the specific job of resetting the body clock after crossing time zones, and only if the timing is right. A Cochrane systematic review found that nine of ten randomised trials showed melatonin reduced jet lag when taken close to the destination's bedtime, and the American Academy of Sleep Medicine rates it a first-tier treatment for jet lag disorder [s1] [s2]. Taken at the wrong time of day, the same review found, melatonin can delay adjustment rather than speed it [s1].

What jet lag is, and why melatonin is a plausible fix

Jet lag results from the body's internal rhythms being out of step with the day-night cycle at the destination [s1]. Melatonin is a hormone the pineal gland releases in the evening, and it is one of the signals that sets the body's daily rhythm — which is the mechanistic reason a timed dose might help realign that rhythm faster than it would drift on its own [s1]. That is theory; the useful question is what the trials show.

What the Cochrane review found

The review pooled the ten randomised trials that existed in airline passengers, cabin crew, and military personnel, all comparing oral melatonin against placebo (one also compared it against the sleeping pill zolpidem) [s1]. Nine of the ten trials found that melatonin, taken close to the target bedtime at the destination — the review specifies 10pm to midnight — decreased jet lag from flights crossing five or more time zones [s1]. The reviewers described the effect as "remarkably effective," a strong phrase for a Cochrane review, and estimated a number needed to treat of 2, meaning roughly one in two travellers benefited who would not have on placebo [s1].

Two details matter for anyone reading the marketing on a melatonin bottle. First, more is not better: daily doses between 0.5 and 5mg were similarly effective, and doses above 5mg appeared no more effective than that [s1]. People fell asleep faster and slept better on 5mg than on 0.5mg, but the jet-lag benefit itself did not keep climbing with dose [s1]. Second, a 2mg slow-release formulation was relatively ineffective, which the reviewers read as a sign that a short-lived, higher peak of melatonin works better than a slow trickle [s1].

The benefit was larger the more time zones were crossed, and smaller for westward flights than eastward ones [s1]. Travellers crossing two to four zones can use it if needed, but the clearest evidence is for five or more zones, especially eastward, and especially in people who have had jet lag on previous trips [s1].

The timing catch, which is the whole story

The single most important finding for a traveller is not that melatonin works — it is that mistiming it backfires. The review states plainly that if melatonin is taken at the wrong time, early in the day, it is liable to cause sleepiness and delay adaptation to local time [s1]. A pill that can either advance or delay the body clock depending on when it is swallowed is not a sleep aid you take whenever you feel tired; it is a circadian signal, and the direction of its effect depends on timing. That distinction is why a guideline recommendation exists at all.

The American Academy of Sleep Medicine's practice parameters — the formal position of the US sleep medicine profession — grade timed melatonin administration for jet lag disorder at their highest recommendation level, "Standard" [s2]. Alongside it, the same guideline lists specifically dosed and timed light exposure as an option, and prescribed sleep schedules as an option; sleeping pills may be an option for jet-lag-induced insomnia, and stimulants an option for daytime alertness, though the guideline flags that stimulants carry risks that must be weighed first [s2]. In other words, the evidence-based toolkit for jet lag is timed melatonin, timed light, and a planned sleep schedule — not any single product.

The limits and the safety notes

The Cochrane review is old — its trials are small, and it calls for systematic study of melatonin's pharmacology and for routine quality control of melatonin products, because supplement melatonin is not regulated as a drug in many countries and the amount in a capsule may not match the label [s1]. It also flagged two specific safety concerns from case reports outside the trials: people with epilepsy, and people taking the blood thinner warfarin, may come to harm from melatonin, and both interactions need proper study [s1]. The incidence of other side effects in the trials was low [s1].

None of this is a dosing instruction, and this article does not offer one. The reason melatonin for jet lag is worth writing about is that it is one of the few cases in consumer wellness where the evidence is better than the hype rather than worse — provided the reader understands that timing, not dose, is what determines whether it helps or hurts, and that a clinician is the right person to sort that out for anyone with epilepsy, on anticoagulants, or unsure when their body clock actually sits.