They spend about one-third of their time asleep, suggesting that most people need about eight hours of sleep each day. This slight imprecision, despite our living on a planet that rotates on its axis every 24 hours, is thought to help humans and other living things adapt to seasonal changes in daylight. Sleeping and waking comprise the most prominent circadian rhythm, but there are hundreds of others that follow a similar schedule, affecting virtually every bodily function studied. Humans are designed for daytime activity and nighttime rest. Additionally, there is a natural peak in sleepiness at mid-day, the traditional siesta time.
Light signals coming through the eyes reset the circadian cycles each day.
These signals travel to a tiny cluster of nerves in the hypothalamus in the center of the brain, the body's master clock. This clock takes its name from its location, just above the optic chiasm, a major junction for nerves transmitting information about light from the eyes. The approach of dusk each day prompts the SCN to signal the nearby pineal gland to produce the hormone melatonin. Melatonin is thought to act as the body's time-setting hormone. It is not thought to trigger sleepiness.