Sufficient and sound sleep is a human need as basic as food, vital to emotional and physical well being. In recent years, scientists have made great strides in identifying patterns and functions of brain activity in sleep.

The daily cycle of life, which includes sleeping and waking, is called a circadian (meaning "about a day") rhythm, commonly referred to as the biologic clock. Hundreds of bodily functions follow biologic clocks, but sleeping and waking are the most prominent circadian rhythms.

Light signals coming through the eyes reset the circadian cycles each day. The response to light signals in the brain is an important key factor in sleep and in maintaining a normal circadian rhythm.

Light signals travel to a tiny cluster of nerves in the hypothalamus in the center of the brain, the body's master clock, which is called the supra chiasmatic nucleus or SCN. This nerve cluster takes its name from its location, which is just above (supra) the optic chiasm. The optic chiasm is 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 (named so because it resembles a pine-cone) to produce the hormone melatonin.

Melatonin is an important hormone released in the brain that some experts believe is critical for the body's time-setting. The longer a person is in darkness the longer melatonin is secreted. Levels drop after staying in bright light. Research is ongoing to determine if high levels of melatonin cause sleep regardless of whether it is dark.

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