The need for sleep is regulated by what is called circadian rhythm, which is the body’s natural process of regulating activity in a 24-hour cycle, including the times that you fall asleep and wake up.

Circadian rhythm is controlled by a “master clock” comprised of a group of cells in the brain called the “suprachiasmatic nucleus,” or SCN.

The SCN consists of about 20,000 nerve cells in the hypothalamus, which is the area of the brain located just above where the optic nerves of the eyes cross.

This nerve center controls your brain’s production of melatonin, the hormone that makes you sleepy.

When night falls and there is less light, this nerve center orders the brain to make more melatonin, which makes you feel drowsy.

Melatonin levels vary at different times of the day. Research shows that for most people, the strongest sleep drive comes between 2 and 4 a.m., and again in the afternoon between 1 p.m. and 3 p.m.

However, there is a good deal of variation, depending on whether you are a “morning” or “evening” person.