Sleep – A Dynamic Activity
Until the 1950s, most people thought of sleep as a passive, dormant part of our daily lives. We now know that our brains are very active during sleep. During sleep, we usually pass through five stages of sleep: stages one, two, three, four, and REM (rapid eye movement) sleep. These stages progress in a cycle from stage one to REM sleep. Then the cycle starts over again with stage one.
We spend almost fifty percent of our total sleep time in stage two sleep, about twenty percent in REM sleep, and the remaining thirty percent in the other stages. Infants, by contrast, spend about half of their sleep time in REM sleep.
During stage one, which is light sleep, we drift in and out of sleep and can be awakened easily. Our eyes move very slowly, and a sensation of starting to fall followed by a sudden contractions of muscles may be experienced. These sudden movements are similar to the “jump” we make when startled.
When we enter stage two sleep, our eye movements stop, and our brain waves become slower. In stage three, extremely slow brain waves called delta waves begin to appear, interspersed with smaller, faster waves.
By stage four, the brain produces delta waves almost exclusively. It is very difficult to wake someone during stages three and four, which together are called deep sleep. There is no eye movement or muscle activity. People awakened during deep sleep do not adjust quickly and often feel groggy for several minutes after they wake up. Some children experience bedwetting, night terrors, or sleepwalking during deep sleep.
When we switch into REM sleep, our breathing becomes rapid, irregular, and shallow. Our eyes jerk rapidly in various directions, and our limb muscles become temporarily paralyzed. Our heart rate increases, and our blood pressure rises. When people awaken during REM sleep, they often describe bizarre and illogical dreams.
The first REM sleep period usually occurs about seventy to ninety minutes after we fall asleep. A complete sleep cycle takes ninety to one-hundred-ten minutes on average. The first sleep cycles each night contain short REM periods and a long period of deep sleep. As the night progresses, REM sleep periods increase in length, while deep sleep decreases. By morning, people spend nearly all their sleep time in stages one, two, and REM.