Melatonin: Much More Than You Wanted To Know | Slate Star Codex

I take melatonin very regularly via REM Caps.

Melatonin works on both systems. It has a weak “hypnotic” effect on Process S, making you immediately sleepier when you take it. It also has a stronger “chronobiotic” effect on the circadian rhythm, shifting what time of day your body considers sleep to be a good idea. Effective use of melatonin comes from understanding both these effects and using each where appropriate.

This read goes into more depth on the uses for melatonin.