A few cautions: Melatonin is considered generally safe for short-term use. However, there have been concerns about risks of bleeding (especially in people taking blood-thinners like warfarin). There also is increased risk of seizure, particularly in children with brain disorders.

http://www.webmd.com/vitamins-and-supplements/lifestyle-guide-11/natural-good-sleep-tips-on-melatonin-valerian

Also, risky for diabetics?