Many transmeridian travellers, including top athletes, regularly take melatonin to reduce negative jet lag symptoms. Standard (rather than individually tailored) doses are often used. We examined the effects of a standard dose of melatonin on the body temperature rhythms of 12 elite biathletes (eight men and four women) after an eastward transmeridian flight to an international competition. The different effects on body temperature rhythms in men versus women underscore the need for personalized dosing schedules to avoid potential undesirable consequences.