>Melatonin administration has been found to increase hair growth in a variety of mammals

>In a randomized double-blind study of 40 women with hair loss, melatonin solution applied to the scalp increased hair growth significantly relative to placebo

Seems like a positive. The article is suggesting that by helping regulate the circadian rhythm, the use of melatonin can potentially: extend lifespan, prolong fertility, reduce cancer incidence, reduce obesity and insulin resistance, improve neurological recovery from brain injury, improve skin resistance to damage (such as from UV rays) and increase hair growth.

Unfortunately, this is unlikely to affect male pattern hair loss.