Sleep Cycles and Illness

The clock in your brain doesn’t just take cues from light, but from the hormone melatonin as well. Every night, about two to three hours before you conk out, your brain starts to secrete melatonin in response to darkness. Taking a melatonin supplement in the evening will advance your internal clock and make it possible to fall asleep earlier; taking it in the morning will do the opposite. (You might assume this would make you even more tired during the day but it won’t; you could think of it as tricking your brain into believing you slept longer.)

Yes, Your Sleep Schedule Is Making You Sick by Richard A. Friedman