Sleep Sleep is one of those funny things about being a human being—you just have to do it. Have you ever wondered why? And what about the crazy dreams, tike the one where a bad person is chasing you and you can't run or yell. Does that make any sense? Characteristics of Sleep We all know how sleep looks—when we see someone sleeping, we recognize the following characteristics: -If possible, the person will lie down to go to sleep. -The person's eyes are closed. -The person doesn't hear anything unless it is a loud noise. -The person breathes in a slow, rhythmic pattern. -The person's muscles are completely relaxed. If sitting up, the person may fall out of his or her chair as sleep deepens. -During sleep, the person occasionally rolls over or rearranges his or her body. This happens approximately once or twice an hour. This may be the body's way of making sure that no part of the body or skin has its circulation cut off for too long a period of time. In addition to these outward signs, the heart slows down and the brain does some pretty frightening things. In other words, a sleeping person is unconscious to most things happening in the environment. The biggest difference between someone who is asleep and someone who has fainted or gone into a coma is the fact that a sleeping person can be aroused if the stimulus is strong enough. If you shake the person, yell loudly or flash a bright light, a sleeping person will wake up. For any animal living in the wild, it just doesn't seem very smart to design in a necessary eight-hour period of near-total unconsciousness every day. Yet that is exactly what evolution has done. So there must be a pretty good reason for it! Who Sleeps? Reptiles(爬行动物), birds and mammals all sleep. That is, they become unconscious to their surroundings for periods of time. Some fish and amphibians reduce their awareness but do not ever become unconscious like the higher vertebrates(脊椎动物) do. Insects do not appear to sleep, although they may become inactive in daylight or darkness. By studying brainwaves, it is known that reptiles do not dream. Birds dream a little. Mammals all dream during sleep. Different animals sleep in different ways. Some animals, like humans, prefer to sleep in one long session. Other animals (dogs, for example) like to sleep in many short bursts. Some sleep at night, while others sleep during the day. Sleep and the Brain If you attach an electroencephalograph(脑电图仪) to a person's head, you can record the person's brainwave activity. A waking and relaxed person generates alpha waves, which are consistent vibrations at about 10 cycles per second. An alert person generates beta waves, which are about twice as fast. During sleep, two slower patterns called theta waves and delta waves take over. Theta waves have oscillations in the range of 3.5 to 7 cycles per second, and delta waves have oscillations of less than 3.5 cycles per second. As a person falls asleep and sleep deepens, the brainwave patterns slow down. The slower the brainwave patterns, the deeper the sleep—a person deep in delta wave sleep is hardest to wake up. REM Sleep At several points during the night, something unexpected happens—rapid eye movement (REM) sleep occurs. Most people experience three to five intervals of REM sleep per night, and brainwaves during this period speed up to awake levels. If you ever watch a person or a dog experiencing REM sleep, you will see their eyes flickering back and forth rapidly. In many dogs and some people, arms, legs and facial muscles will twitch(抽搐) during REM sleep. Periods of sleep other than REM sleep are known as NREM (non-REM) sleep. REM sleep is when you dream, ff