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★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
Short trips under five time zones were never tested with melatonin
Every trial crossed five to twelve time zones, mostly flying east, and none compared short trips with long ones.
What the study found
Five to twelve time zones: the only trips melatonin was tested on
Its jet lag trials flew travellers across five to twelve time zones, most of them eastward, and none compared short trips with long ones. So the advice to use it for five or more zones marks where it was tested, not a threshold anyone found.
The caveat · Few trials flew west
Benefit after westward flights rests on fewer trials: two in one pooling, four in another. Two trials found benefit after westward trips of twelve or more time zones.