Jet-lag gains shrank from the first melatonin trial to the largest
Counting two later trials that found no benefit, one of 257 people, melatonin still came out ahead, by about half the margin of a pooling without them.
What the study found
The biggest effect came in the first trial, near zero in the largest
The first trial of melatonin for jet lag, 17 people in 1987, found the largest benefit of any. Trials in 1999 and 2000, one of them the largest at 257 people, found close to none. Counting them, melatonin still comes out ahead, by about half the margin of a pooling that leaves them out.
The caveat · Largest trial: just five days away
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.
What we don't know
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.