Sufficient and restful sleep is a human need as basic as food, vital to emotional and physical well being. Although one-third of the average person's life is spent sleeping, many of the physical and chemical bases of sleep remain a mystery. Sleep-wake patterns are governed by circadian rhythms, which usually run on 25-hour cycles with two natural daily peak times for sleeping, at night and at mid-day, the traditional siesta time. As light fades, the cells in the retina of the eye pass a message directly to a cluster of nerve cells correctly called the suprachiasmatic nucleus (SNA) (commonly called the circadian clock) which is located in the hypothalamus deep in the center of the brain. The SNA then signals the pineal gland located in the hypothalamus to produce the hormone melatonin, which is manufactured from tryptophan. Melatonin is the brain's messenger for changes in light; it causes a drop in body temperature and sleepiness. Researchers have also located a cluster of nerve cells close to the circadian clock that may act as a master switch to deactivate three major neurotransmitters (chemical messengers) involved in arousal; these are histamine, norepinephrine, and serotonin.

Sleep normally progresses through several stages, which are repeated in sequence several times during the night. These are categorized as rapid eye movement (REM) sleep and non-REM sleep, which has four stages. Stage 1 (light sleep) occurs in the transition between wakefulness and Stage 2 (so-called true sleep). Stage 3 and 4 are collectively known as deep sleep and are characterized by slow brain activity. Research is now suggesting that non-REM sleep may play a role in bolstering the immune system and may also be tied to the rhythms of the digestive tract. It is during non-REM sleep that growth hormone is released. REM first occurs about 90 minutes after falling asleep and then about three to five times thereafter; during REM sleep muscle tone is greatly reduced, but brain waves show the same active frequency as when a person is awake. This is the dreaming phase of sleep, necessary for long-term emotional well being. Scientists now believe that REM sleep may also help bolster memory. Although the benefits of sleep are not precisely known, interrupted sleep at night can lead to daytime impairments.

Sleep cycles and sleep needs vary greatly on an individual basis. Most adults need seven to eight hours of sleep per night, usually beginning in the late evening and ending at sunrise. In elderly people, sleeping habits often become erratic, causing midday dozing and midnight wakefulness. Some people who are hyperactive or have tense and driven natures tend to sleep less. Many highly creative and productive people sleep less than five hours per night without being sleep deprived. A consistent pattern rather than the number of hours which one sleeps, seems to be the key to a healthy sleep cycle.