SLEEP IS STILL SURROUNDED BY MYSTERY
Sleep is a natural process, and although a lot have been written about the subject, it is still surrounded by mystery. It is used by some as an escape from the world, and regarded by others as an irritating waste of time: some people get by on very little, others claim they cannot exist without at least ten hours, but nobody can do without sleep completely. 
Our night’s sleep does not just consist of a steady phase of gradually deepening sleep. It alternates between two stages: Non-dreaming or ordinary sleep, and REM (rapid eye movement) or dreaming sleep. As soon as we fall asleep, we go straight into non-dreaming sleep for an hour or so, then into REM sleep for about 15 minutes, then back into non-dreaming sleep. It alternates in this way for the rest of the night, with non-dreaming sleep tending to last longer at the beginning of the night. Non-dreaming sleep occupies three-quarters of our night’s sleep, about a quarter of it deep and the rest fairly light. 
It is widely believed that sleep repairs the body and makes good the damage caused by being awake. However, its main function is to refresh the brain. Experts believe that probably only about two-thirds of our sleep is necessary for repairing and refreshing the brain, with the most valuable sleep coming in the first few hours of the non-dreaming period, the last few hours of sleep are not so essential. The brain can manage quite well with reduced sleep as long as it is uninterrupted sleep. 
The quality of sleep is important. A study conducted in the USA looked at short sleepers, who slept for 5.5 hours on average, and long sleepers, who had 8.5 hours or more. It is discovered after a variety of tests that the long sleepers were poor sleepers, had twice as much REM sleep as the short sleepers, appeared to sleep longer to make up for poor sleep, and did not wake up in the morning refreshed. Similarly, people who sleep deeply do not necessarily get a better quality of sleep than shallow sleepers. Deep sleepers can feel tired the following day, so six hours of good sleep is worth more than eight hours of troubled sleep.

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