Scientists can't answer the question why people sleep. The function of sleep has become a fascinating mystery in brain science. Many researchers put forward their own views from different angles. The most common view is that sleeping is to eliminate physical fatigue and make up for the loss of a hard day. The evidence of the view of physical recovery is that in the first few hours of sleep, the pituitary gland at the bottom of the brain releases a large amount of growth hormone, which can promote the metabolism of protein in the body, thus promoting the growth and repair of tissues in the body. Dr Peter garrick of the Clinical Nutrition and Metabolism Research Organization in London holds a negative attitude. He believes that the biggest influence on protein metabolism in the body is diet. Tissue protein increases when eating and decreases when fasting. The main reason why protein's metabolism changes at night is not sleep itself, but that people don't eat at night. Garrick thinks that no matter what kind of physical or mental work a person is engaged in, no matter how tired he is, even if he doesn't sleep for 8 ~ 1 1 day in a row, his body function is still intact. In a sleep experiment, the researchers examined the urine of people who didn't sleep for 3-5 days and found that the nitrogen content in their urine changed less than 1%. Nitrogen is a natural index of protein metabolism in the body, so it can be judged that the physiological function of these subjects has not decreased. In addition, people who voluntarily reduced their sleep for two and a half hours did not have any pathological manifestations after one year, and they were not tired because of reduced sleep. The most convincing view is Professor Hartman, director of the Sleep Laboratory of Boston Mental Health Center. He believes that sleep has two functions: the first is to eliminate physical fatigue, and the second is to eliminate mental fatigue. In his view, there is no doubt about eliminating physical fatigue, and the function of eliminating mental fatigue was discovered in his research on short sleepers in recent years. Hartman once compared short sleepers who slept only 4 hours a night with long sleepers who slept 8-9 hours a night. He found no physiological difference between short sleepers and long sleepers. Their height, weight and even intelligence are the same. However, their respective psychological states are quite different. Long sleepers are always worried, while short sleepers are extremely optimistic. Hartman believes that the elimination of physical fatigue mainly occurs in the so-called slow-wave sleep in the early stage of sleep, that is, from drowsiness, shallow sleep to deep sleep and after deep sleep. Short sleepers and long sleepers experience almost the same proportion of diffuse sleep. The recovery of mental fatigue mainly occurs during REM sleep after deep sleep. Because long sleepers have more troubles than short sleepers, they need longer time to recover from mental fatigue than short sleepers. Short sleepers are just the opposite. Hartman thinks this is why Pauline, who sleeps only two hours a day, doesn't have REM sleep. Another view is that the main function of sleep is to restore brain fatigue. They enumerated the fact that dolphins sleep. There is a dolphin that can rest its body when it is awake, but it still needs to sleep. The way to sleep is very strange. Within an hour, half of their brains take turns sleeping and the other half stay awake. According to the measurement, half of the sleeping brain is dominated by deep sleep, so it seems that the dolphin's sleep is mainly to restore brain fatigue, but has nothing to do with physical recovery. When dolphins sleep, their bodies are still swimming in the water. Dr Wilkinson, a practical psychologist at Cambridge University in England, dismissed the dolphin example. He believes that animal sleep patterns can't explain people's sleep. Animals and humans have different evolutionary environments, so the sleep mechanism can't be compared. Wilkinson and others found that at the beginning of the experiment of banning sleep, people who don't sleep can still be refreshed with the rising of the sun, which is no different from usual in many ways. According to statistics, 75% of people have normal EEG response after a sleepless night. Even in the experiment of long-term sleep ban, the central nervous system function of the subjects showed no signs of disorder. There is a view that people's sleep has no effect on restoring physical strength at all, but an unnecessary instinctive behavior for staying overnight. It is still difficult to determine which of the above viewpoints is right or wrong. Because the neural circuits that control sleep in the brain are extremely complex, it is very difficult to reveal the secret of sleep.