Does melatonin actually help jet lag? Less than the first trial found

A ranking says which treatment the evidence supports better, not how much it helps. Of the five studies a 2014 review accepted, three tested sleeping pills and one tested light. Melatonin's rating rests on the fifth, an earlier review of ten trials, graded low quality because doses and schedules varied. A later reading of the head-to-head found melatonin alone did not clearly lower overall jet lag.

Slow-release caffeine sped the hormones' return just as much in one of the studies. In a trial of 31 people whose days were full of irregular daylight, melatonin did not speed the body-temperature rhythm's shift. The two smallest studies, of 6 and 8 people, described no placebo group.

The pooled scores come from four small trials of 17 to 52 people, published from 1987 to 1992. Two later trials that found no benefit, one of them the largest at 257 people, were left out of that pool. A re-analysis that counts them finds about half the effect of a pooling that leaves them out. Ratings varied so much between people that their spread often exceeded 90% of the average score, so the 40% is a gap between group averages, not what each traveller felt. Scores were compared after the flight, not as change from how each person felt before it.

The decline was shown, not tested, and it runs alongside other changes: the later trials were larger, rated jet lag before the flight, or used a new scale. The 257 travellers had spent only five days away, so their body clocks had less to readjust on the way home. The reviewer who added the two null trials to the pool co-wrote one of them.

Every pooling of jet-lag ratings favours melatonin, but how much it helps turns on two later trials that found no benefit and were left out of the bigger estimates. Whether they were fair tests is disputed; no newer trial has settled it.