First described in 1953 by Nathaniel Kleitman and Eugene Aserinsky, rapid eye movement (REM) sleep is also called active sleep because the EEG ( electroencephalogram) patterns in this stage are similar to the patterns during the awake stage. The four stages of slowwave, or non-REM, sleep are accompanied by deep breathing, a relatively slow heartbeat, and lowered blood pressure.
In contrast, levels of physiological arousal during REM sleep resemble those of the waking state. In some ways, however, people are more deeply asleep during the REM stage than at other times: the major muscle groups go limp in a sort of paralysis, and people are hardest to waken during REM sleep.
The contradictions between the active, “awake” features of REM sleep and its soundness have caused some people to refer to REM sleep as “paradoxical sleep.” At birth about 50 percent of all sleep is REM sleep, but by the age of 10 this figure drops to 25 percent.
In the course of a night, periods of REM sleep occur every 90 to 100 minutes, becoming longer as the night progresses, in contrast to the deeper stage four sleep, most of which occurs early in the night. About 80 percent of the time, people awakened from REM sleep will say they have been dreaming, while those awakened during other sleep stages rarely report dreams.
Experiments have shown that people repeatedly awakened during the REM stage for several nights will compensate by spending twice as much time in REM sleep the first night they are left alone, an observation that has led to much speculation about the role of this type of sleep.
Some researchers have hypothesized that REM sleep strengthens neural connections in the brain,a theory supported by the fact that infants and children, whose brains are still developing, require larger amounts of REM sleep than adults. It has also been suggested that REM sleep may be linked to a specific neurotransmitter, norepinephrine, which helps maintain alertness when people are awake.
In addition, REM sleep has been investigated in connection with learning and memory in studies that showed decreased retention of learned skills in persons who were deprived of REM sleep. However, a contrasting (and controversial) theory maintains that the REM stage is a way for the body to “empty” the brain so that its neural networks do not become overloaded.