Introduction

• This nerve cluster takes its name from its location, which is just above (supra) the optic chiasm, which 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 thought to act as the body's time-setting hormone. The longer a person is in darkness the longer the duration of melatonin secretion. Secretion can be diminished by staying in bright light. Melatonin also appears to trigger the need to sleep.

Sleep Cycles

Sleep consists of two distinct states that alternate in cycles and reflect differing levels of brain nerve cell activity:

Non-Rapid Eye Movement Sleep (NonREM). NonREM sleep is also termed quiet sleep. NonREM is further subdivided into three stages of progression:

• Stage 1 (light sleep)
• Stage 2 (so-called true sleep)
• Stage 3 to 4 (deep "slow-wave" or delta sleep)

With each descending stage, awakening becomes more difficult. It is not known what governs NonREM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones, may be important for deep sleep.

Rapid Eye-Movement Sleep (REM). REM sleep is termed active sleep. Most vivid dreams occur in REM sleep. REM-sleep brain activity is comparable to that in waking, but the muscles are virtually paralyzed, possibly preventing people from acting out their dreams. In fact, except for vital organs like lungs and heart, the only muscles not paralyzed during REM are the eye muscles. REM sleep may be critical for learning and for day-to-day mood regulation. When people are sleep-deprived, their brains must work harder than when they are well rested.

The REM/NREM Cycle. The cycle between quiet (NonREM) and active (REM) sleep generally follows this pattern:

• After about 90 minutes of NonREM sleep, eyes move rapidly behind closed lids, giving rise to REM sleep.
• As sleep progresses the NonREM/REM cycle repeats.
• With each cycle, NonREM sleep becomes progressively lighter, and REM sleep becomes progressively longer, lasting from a few minutes early in sleep to perhaps an hour at the end of the sleep episode.