Rapid eye movement (REM) is the stage of sleep during which the most vivid (though not all) dreams occur. During this stage, the eyes move rapidly, and the activity of the brain's neurons is quite similar to that during waking hours. It is the lightest form of sleep; people awakened during REM usually feel alert and refreshed.
During a night of sleep, a person usually has about four or five periods of REM sleep, which are quite short at the beginning of the night and longer at the end. It is common to wake for a short time at the end of a REM phase. The total time of REM sleep per night is about 90-120 minutes.
Physiologically, certain neurons in the brain stem, known as REM sleep-on cells, are particularly active during REM sleep, and are probably responsible for its creation. The release of certain neurotransmitters, the monoamines (norepinephrine, serotonin and histamine) is completely shut down during REM. For this reason, the motor neurons are not stimulated by the brain's activity and the body's muscles don't move. The heart rate and breathing rate are irregular during REM sleep, again similar to the waking hours. Body temperature is not well regulated during REM, and it approaches the surroundings' temperature. Erections of the penis or clitoris are also common during REM. In fact, REM sleep is so physiologically different from the other phases of sleep that the others are collectively referred to as non-REM sleep.
The function of REM sleep is not understood; several theories have been advanced. According to one theory, memories are consolidated during REM sleep. However, the evidence seems scant; in people that have no REM sleep (because of brain damage or drugs) memory functions are not affected. Another theory has it that the monoamine shutdown is required so that the monoamine receptors in the brain can recover to regain full sensitivity. Indeed, if REM sleep is repeatedly interrupted, the person will "make up" for it with longer REM sleep at the next opportunity. Interrupting REM sleep can improve certain types of depression, and depression appears to be related to an imbalance of certain neurotransmitters. According to a third theory, the REM sleep of newborns provides the neural stimulation necessary to form mature neural connections; hence maturely born animals don't need much of it. Supporting this theory is the fact that the amount of REM sleep decreases with age.
REM sleep also occurs in other mammals. It appears that the amount of REM sleep per night in a species is closely correlated with the developmental stage of newborns. The platypus, whose newborns are completely helpless and undeveloped, has 8 hours of REM sleep per night; in the dolphin, whose newborns are almost completely functional at birth, there is almost no REM sleep.
The association between dreaming and REM sleep was discovered by Eugene Aserinsky and Nathaniel Kleitman in 1953.
- Jerome M. Siegel: Why We Sleep. Scientific American, November 2003, pp. 92-97