Melatonin

The pineal gland secretes melatonin.

Chief among them, says Buchanan, is melatonin, a hormone stimulated by darkness to play a key role in regulating the sleep-wake cycle and cell division. Scientists say reduced melatonin production is a likely factor in the significantly higher cancer rates in night workers, and they propose that modern night lighting contributes to the larger incidence of some cancers in the industrial world.

Melatonin helps to regulate circadian rhythms.

How exactly melatonin relates to sleep is still not understood.

Melatonin production can be delayed or arrested by even brief flashes of bright light entering the eye, research has shown.