Sleep is a state during which people or animals are unaware of their surroundings. Sleep, unlike a coma or hibernation, is easily ended. A sleeping person or animal can be awakened quickly by, for example, a loud noise or a bright flash of light. All human beings and most animals require a certain amount of sleep each day.
People typically sleep lying down with eyes closed. When a person falls asleep, muscles relax, the heartbeat slows, and breathing becomes slow and regular. A sleeping person usually changes position and awakens briefly several times a night.
Human sleep patterns.
Most people sleep at night, but others, such as those who work at night, sleep during the day. In some cultures, particularly in Spain and Latin America, sleep periods have traditionally been divided into two parts. Some people in these cultures take an afternoon nap in addition to sleeping at night. But, they usually get the same amount of sleep overall as people who do not take a midday nap.
Newborn babies sleep for brief periods throughout the day and night. By 2 to 3 months of age, most babies sleep through the night and take naps during the day. By 6 years of age, most children have given up daytime naps. Four-year-olds average from 10 to 14 total hours of sleep a day, while 10-year-olds average from 9 to 12 total hours of sleep a day. The average sleep need for a human adult is approximately 81/2 hours. Some people need more sleep, and others need less.
Older adults need as much sleep as they did when they were younger. However, many older adults have trouble sleeping. Certain sleep disorders, such as insomnia or sleep apnea, are more common among older adults (see Insomnia; Sleep apnea). Scientific research indicates that the amount of sleep needed by an adult is biologically determined and cannot be changed.
The structure of sleep.
Scientists study sleep with an instrument called an electroencephalograph (see Electroencephalograph). The brain gives off electrical waves when a person is asleep or awake. An electroencephalograph measures and records these waves. The brain of an awake, relaxed person gives off about 10 small waves per second.
Sleep in human beings is composed of two states called rapid eye movement (REM) sleep and non-rapid eye movement (nonREM) sleep. Sleepers spend most of the night in nonREM sleep. NonREM sleep is divided into four sleep stages. It is easy to awaken a person from the first and second sleep stages, but more difficult to awaken someone from deeper sleep stages. As a sleeper goes through these stages, brain waves become slower and larger, and sleep becomes deeper. During the third and fourth stages, which involve the deepest sleep, the brain gives off about four waves per second.
During REM sleep, the brain is extremely active, and the brain waves are similar to those of an individual who is awake. REM sleep occurs about every 90 minutes and lasts from 5 to 30 minutes. Periods of REM sleep in the later part of the night last longer than earlier periods. Most dreams occur during REM sleep, and if a sleeper is awakened from REM sleep, that person is more likely to remember the content of a dream.
Sleep in animals.
Scientists study sleeping animals by the same methods that they study sleeping human beings. Nearly all animals have some period of rest, although scientists have not identified true sleep in all species. Among the vertebrates (animals with backbones), reptiles, birds, and mammals experience true sleep, indicated by changes in brain wave patterns. Most reptiles do not experience periods of REM sleep, and most species of birds have only brief ones. All mammals have periods of REM sleep and deep sleep.
Different species of reptiles, birds, and mammals have different sleep patterns. Some sleep for many short periods every day, but others sleep for one long period. Some animals are nocturnal—that is, they are active at night and sleep during the day. Some mammals, such as cattle, can sleep while standing up, but they dream only while lying down. Insects, spiders, and other invertebrates (animals without backbones) have daily periods of reduced activity. However, these animals do not show the sudden decrease in their ability to respond to their surroundings that is characteristic of true sleep. These organisms also do not show any of the brain wave changes that are associated with sleep in vertebrates.
What happens without sleep.
People who are deprived of sleep have difficulty concentrating, especially when working on boring or routine tasks. They may be able to perform tasks fairly well for short periods, but they are easily distracted. After a while, they become increasingly likely to make mistakes. However, many sleep-deprived people can still feel alert and do well on more interesting tasks.
Sleepless people may experience periods of dozing off for a few seconds or more. Such incidents can occur even during an activity, such as reading or driving a car. A person may not even be aware of these short periods of sleep.
People who go without sleep for more than three days have great difficulty thinking, seeing, and hearing clearly. Some have hallucinations, during which they see things that do not really exist. They can confuse daydreams with real life and often lose track of their thoughts in the middle of a sentence.
People who get enough sleep are more able to stay awake and alert during the day, even during quiet and routine tasks. Many people do not get enough sleep on a regular basis. Some people who do not get enough sleep find it difficult to wake up in the morning and can have trouble staying awake during the day.
The need for sleep.
Scientists know that sleep helps restore the body and the brain. Sleep may also help make memories of recent events more permanent. Scientists believe that deep sleep may be necessary for building new protein and ridding the brain of waste proteins and other debris. These materials can build up and become toxic to the brain if they are not flushed away during sleep. REM sleep may be especially important for maintaining such mental activities as learning, reasoning, and emotional control. However, scientists are still seeking answers to many questions about the need for sleep.
