"It really amazes me that melatonin is available in any pharmacy," Bentley said. "It is a powerful hormone, and yet people don't realize that it's as 'powerful' as any steroid. I'm sure that many people who take it wouldn't take steroids so glibly. It could have a multitude of effects on the underlying physiology of an organism, but we know so little about how it interacts with other hormone systems."
Popular supplement melatonin found to have broader effects in brain than once thought (2005)
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.
Full essay: http://www.gwern.net/Melatonin