Sleep is a dynamic activity during which the brain continues to work while the body rests.
Sleep has five phases. During the first stage, a person can be easily awakened. At this stage, the eyes move slowly. Often times, when a person is awakened during this stage sudden muscle contractions occur and may give the awakening person the feeling of falling.
At stage 2, eye movement stops and brain waves slow and produce intermittent bursts of rapid waves known as “sleep spindles”. In stage 3, delta waves start to appear. These are extremely slow brain waves interspersed with smaller, faster waves.
During stage 4, only delta waves are produced by the brain. It will most likely take much effort to wake someone whose sleep is at stages 3 and 4, when sleep is deep and there is virtually no more eye movement or muscle activity. People who are awakened during deep sleep experience temporary disorientation. It is also during deep sleep that children experience bedwetting, night terrors and sleepwalking.
REM (rapid eye movement) sleep is the last stage where breathing becomes faster, shallow and irregular. The eyes jerk rapidly in different directions with the muscles of the limbs becoming paralyzed for short periods of time. During this stage, the heart rate increases, blood pressure rises, and penile erections occur in men. Dreams are experienced during REM sleep stage.
These stages repeat, and a complete sleep cycle completes in 90 to 110 minutes on average. The first sleep cycle each night is characterized by short REM periods and long periods of deep sleep. As the night goes by, REM sleep periods become longer while deep sleep periods get shorter. In the morning, people spend their sleep time in stages 1 and 2 and REM stages. According to research, we spend 50 percent of our sleep time in stage 2 sleep, about 20 percent in REM sleep and 30 percent in various other sleep stages. In comparison, infants spend half of their sleep time in REM stages.