Scientists divide sleep into two major types: REM (rapid eye movement) sleep or dreaming sleep, and non-REM or quiet sleep. Surprisingly, they are as different from each other as each one is from waking— yet both may be important for energy.
Non-REM sleep involves three stages. Sleep specialists believe that the last of them—known as deep sleep or slow-wave sleep—is the main time when your body renews and repairs itself. This stage of sleep appears to be the one that plays the greatest role in energy, enhancing your ability to make ATP, the body’s energy molecule. In deep sleep, blood flow is directed less toward your brain, which cools measurably. At the beginning of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissue growth and muscle repair. Researchers have also detected increased blood levels of substances that activate your immune system, raising the possibility that deep sleep helps prepare the body to defend itself against infection.
Someone whose deep sleep is restricted will wake up feeling less refreshed than a person who got adequate deep sleep. When a sleep-deprived person gets some sleep, he or she will pass quickly through the lighter sleep stages into the deeper stages and spend a greater proportion of time there, suggesting that deep sleep fills an essential role in a person’s optimal functioning.
Just as deep sleep restores your body, scientists believe that REM sleep restores your mind, perhaps in part by helping clear out irrelevant information. Studies of students’ ability to solve a complex puzzle involving abstract shapes suggest that the brain processes information overnight; students who got a good night’s sleep after seeing the puzzle fared much better than those asked to solve the puzzle immediately. Other studies, from Harvard Medical School and elsewhere, have found that REM sleep facilitates learning and memory. People who were tested to measure how well they had learned a new task improved their scores after a night’s sleep. If they were prevented from having REM sleep, the improvements were lost. By contrast, if they were awakened an equal number of times from deep sleep, the improvements in the scores were unaffected.
There is also emerging evidence that getting enough REM sleep may help to preserve memory and cognitive function as you age.