There's a bunch of easily found stuff on animal experiments where it shrinks gonads and brains. It clearly turns off fertility in animals. It's well understood as the "winter hibernation" hormone. It's not the "good circadian rhythm night of deep sleep" hormone like people seem to think. It rises to adapt the body to the stress of darkness of winter, it doesn't rise to make you get better rest. It makes the retina more sensitive to light and lightens skin. It's high in seasonally depressed people. Taking a sunny vacation lowers it, and makes you feel better.

Nobody has a melatonin deficiency; you don't need melatonin levels to get restful sleep. If stress hormone levels are low you can fall asleep under bright lights with no melatonin.

I can't say taking 5mg every now and then would be harmful to humans, but when you understand melatonin's essential role as the winter hibernation hormone, and consider the animal experiments, I think you might want to prefer other means to get to sleep. Personally, I find diphenhydramine (benadryl) pretty good for occasional help nodding off. And that's very clearly safe for occasional use.