No serious adverse effects have been associated with melatonin but the number of people involved in trials has been small and no long term data exist. Conclusion On the limited published evidence available, oral melatonin possibly reduces the severity and duration of jet lag. There is little or no data on the product's toxicity or long-term safety and no clear evidence on when or how it should be taken. We cannot recommend the use of melatonin for the management of jet lag without more evidence of its value and while it does not have a product licence to ensure its quality and safety. Brzezinski A. Melatonin in humans.
Light suppresses melatonin secretion in humans.
Arendt J. Melatonin. Arendt J, Aldhous M, English V et al. Some effects of jet-lag and their alleviation by melatonin. Chamberlain K. Effect of melatonin on jet lag after long haul flights. Advances in Pineal Research.