drusillas_rain

As others said, melatonin is the front-end trigger for "wow, it's dark - I should go to sleep." I find that people who cycle hard (ie, their diurnal cycles are more fixed - they find it harder to, say, adjust to jetlag) respond well to melatonin. I respond quite well to it, and basically take it 30 mins before I'm supposed to be asleep. This frequently means as I'm boarding a plane for a late-night flight. ;)