Explain what melatonin has to do with circadian rhythm?

Resets clock: Your internal clock (circadian rhythm) does not run exactly 24 hours. Both sunlight and melatonin produced by the brain 'tell' your internal clock when morning and evening are occurring. This resets your clock so it can be aligned with external 'real world' time. Melatonin lets your internal clock know when the end of the day (bedtime) is occuring.

Light/dark cycle: Melatonin is a hormone released by the pineal gland. This gland is connected to the nerves emanating from your eyes, and when it is dark, the gland releases melatonin. It is thought that melatonin helps you fall asleep. Melatonin is available commercially, and it may help people fall asleep. Melatonin may also help shift the body clock forward or backward, depending on when it is taken.