Sleep is as important as breathing or eating. In fact, people can survive longer without food than they can without sleep. Sleep is vital for giving your body a rest and allowing it to prepare for the next day. It's like giving your body a mini-vacation. Sleep also gives your brain a chance to sort things out. Scientists aren't exactly sure what kinds of organizing your brain does while you sleep, but they think that sleep may be the time when the brain sorts and stores information, replaces chemicals, and solves problems.

The amount of sleep a person needs depends a lot on his age. Babies sleep a whole lot - about 16 or 17 hours a day! But many older people only need about six or seven hours of sleep a night. Most kids between the ages of five and 12 are somewhere in between - they sleep eight to ten hours a night. But the amount of sleep a kid needs really depends on the kid: some find they need a little less sleep, some more.

Skipping one night's sleep makes a person cranky and clumsy. After missing two nights of sleep, a person will have problems thinking and doing things; his brain and body can't do their normal tasks nearly as well. After five nights without sleep, a person will hallucinate (this means seeing things that aren't actually there). Eventually, it becomes impossible for the brain to give its directions to the rest of the body without sleep - the brain needs to spend time in bed and catch its zzzz's!

In the 1950s American physiologists Eugene Aserinsky and Nathaniel Kleitman reported that periods of eye movement and twitching occur during sleep. They named these periods rapid eye movement (REM) sleep. Aserinsky and Kleitman found that when subjects were awakened during REM sleep, they reported vivid dreams. Scientists believe that REM sleep is closely related to wakefulness because brain wave activity during REM sleep is marked by short, rapid wave patterns similar to brain wave activity of the waking state.
Sleep characterized by little or no eye movement is called nonrapid eye movement (NREM) sleep. During NREM sleep, breathing and heart rates slow down, and body temperature and blood pressure often decrease. When awakened from periods of NREM sleep, subjects are much less likely to report vivid, action-packed dreams. Brain wave activity during NREM sleep is dominated by large, slow waves that contrast markedly to the short, rapid wave patterns characteristic of REM sleep and the waking state.

Sleep studies based on EEGs have shown that during a normal night, humans cycle between REM sleep and NREM sleep in very regular patterns. In adults aged 20 to 60, REM sleep occurs about every 90 minutes. In this 90-minute cycle, humans fall into progressively deeper stages of NREM sleep, then cycle back through the stages until they enter REM sleep, and then the cycle begins again. In a normal night, the number of REM periods varies from four to six, depending on the length of the episodes and the total time asleep. REM episodes in the beginning of the night usually last about ten minutes and, during the night, grow progressively longer, lasting up to 30 minutes in the early hours of the morning. Most adults spend about 20 percent of their total sleep time in REM sleep.

Sleep research shows that certain regions of the brain play critical roles in sleep. The brainstem, the portion of the brain just above the spinal cord, is critical in REM sleep control, while the forebrain is particularly important in NREM sleep control.

REM sleep is generated by a region in the brainstem, called the pons, and adjacent portions of the midbrain. Researchers have found that chemical stimulation of the pons will induce very long periods of REM sleep, while damage or injury to this brain region can greatly reduce or even prevent REM sleep. Animal studies have found that some neurons within the pons and midbrain are active only in REM sleep while other neurons in this region are entirely inactive only during REM sleep. Together, these neurons control muscle tone and other aspects of REM sleep. In REM sleep, most muscles in the body are turned off. This lack of muscle tone, called atonia, is particularly complete in the muscles of the back, neck, arms, and legs. Less affected are the muscles that move the eyes and the muscles responsible for breathing.

The combined effect of the sleep-active and sleep-inactive neurons explains why sleepers do not physically act out the vivid dreams they have during REM sleep and instead only twitch or make small movements. Humans with malfunctioning REM sleep-active and REM sleep-inactive systems thrash around in their sleep, often punching their bedmates or hurting themselves as they act out their dreams.

The neurons most critical to NREM sleep control are in the basal forebrain, the region of the brain lying in front of the hypothalamus. Researchers have found that people who have suffered damage or injury to the neurons in the basal forebrain have difficulty falling and staying asleep. Animal studies have shown that this area contains neurons that become most active before and during sleep. Many of these neurons are activated by heat, which explains how a warm bath or a hot day at the beach causes sleepiness.

Amounts of sleep vary significantly with age and even between individuals. Newborns sleep the most-a newborn baby sleeps between 17 and 18 hours a day, spending nearly half of that time in REM sleep. Both REM and NREM sleep decrease with age, and by age five, children sleep between 10 and 12 hours a day, spending about 20 percent of that time in REM sleep.

The average young adult seems to need about 8 hours of sleep per night to function optimally during waking hours. Some people, however, sleep just 6 or 7 hours a night, while others need more than 9 hours to feel rested. The elderly spend less time in deep NREM sleep, and their sleep is more easily interrupted.

Although no one knows for sure why we sleep, there are a number of theories. Sleep may have evolved to protect animals from their predators by reducing their activity during the times when they are most vulnerable.

Research has shown that REM and NREM sleep may serve specific biological functions. Sleep deprivation studies reveal that humans and other animals respond to sleep loss in the same way. When study subjects are deprived of REM sleep, they tend to spend longer periods in REM sleep during their next sleeping period to make up for the loss. REM sleep after deprivation is more intense, with more eye movements per minute than in normal REM sleep. Similarly, subjects deprived of NREM sleep usually spend more time in NREM sleep afterward. EEGs measuring brain activity show that this rebound NREM sleep also differs from normal NREM sleep. This research suggests that the body needs adequate levels of both REM and NREM sleep. This conclusion has led many sleep researchers to believe that the two kinds of sleep serve different biological purposes, although the exact functions remain unclear.

The relationship between maturity at birth and REM sleep suggests that REM sleep plays a role in the development of the brain. REM sleep may have a related function later in life as well. However, that function remains a mystery.