Sleep, natural state of rest characterized by reduced body movement and decreased awareness of surroundings. Sleep is distinguished from other sleeplike states, for instance, hibernation or coma, because it is easily interrupted by external stimulation, such as a loud noise. While the exact purpose of sleep remains a mystery, sleep researchers have made enormous strides in understanding how sleep occurs in humans and other animals, and the nature of sleep disorders. All mammals and birds sleep, but scientists are unsure if reptiles, fish, insects, and other life forms sleep. Total sleep amounts differ greatly across species. In general, large mammals tend to sleep less than small mammals. The giraffe and elephant, for instance, sleep only 2 to 4 hours a day, while bats, opossums, and armadillos sleep 18 hours a day or more. While sleeping, most animals close their eyes and adopt particular positions referred to as sleep postures. Humans typically lie down to sleep, for example, while giraffes kneel and bend their long necks around to rest their heads in the crook of their hind knee. Some animals, such as dolphins, can sleep while they are moving. Scientists measure sleep by placing metal electrodes on the scalp to record the electrical activity of the brain. This procedure, called electroencephalography (EEG), enables sleep researchers to evaluate levels of brain activity at different times during sleep. Researchers use similar electrodes to record a sleeping person's body muscle activity and rate of eye movement.
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.