Stages of sleep were discovered in the 1950s and 60s\u2014when researchers first placed electrodes on the heads of sleepers and recorded electrical activity all night long. Scientists quickly discovered that there were different types of sleep. Deep, or slow wave sleep, preceded periods of lighter sleep. Also, a period of dreaming (REM) always came at the end of a \u201csleep cycle.\u201d Sleep cycles in humans last an hour and a half, which is why we tend to sleep in multiples of 90 minutes. Count the number of hours you sleep tonight and you will see that this is true.<\/p>\n
Today it is generally accepted that deep sleep repairs the body, while REM repairs the mind. Interestingly, these same sleep patterns\u2014deep sleep, light sleep, and REM\u2014have been found in every warm-blooded creature on earth. In addition to repairing the brain, REM is strongly implicated in growing it. All neurologically complex, warm-blooded creatures have more REM when they are young than when they are mature.<\/p>\n
Despite recent advances in understanding sleep, we still know very little about it\u2014especially REM. The primary reason sleep is elusive to us is because psychologically, by definition, it is an unconscious state. Things still happen to us during sleep\u2014light sleep, deep sleep, and dreaming\u2014but without consciousness, there\u2019s no \u201cI\u201d there to experience it in the present tense. This is why we always have to \u201clook back\u201d on our experience of sleep. \u201cWe\u201d weren\u2019t really there to experience it the first time around.<\/p>\n
When we are unconscious, our memory suffers as well. This explains why we typically don\u2019t remember much\u2014aside from the occasional dream\u2014from our usual seven or eight hours of sleep. When we are unconscious we also lose our sense of time. Why? Because there\u2019s no one \u201cthere\u201d to notice its passing.<\/p>\n