REM sleep

Rapid eye movement (REM) is the stage of sleep characterized by rapid saccadic movements of the eyes. During this stage, the activity of the brain's neurons is quite similar to that during waking hours. Most of the vividly recalled dreams occur during REM sleep. It is the lightest form of sleep, and people awakened during REM usually feel alert and refreshed. REM sleep is so physiologically different from the other phases of sleep that the others are collectively referred to as non-REM sleep.

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 for an adult. However, the relative amount of REM sleep varies considerably with age. A newborn baby spends more than 80% of total sleep time in REM mode, while people over 70 years old spend less than 10%. The average is 20%.



Physiology of REM sleep
Physiologically, certain neurons in the brain stem, known as REM sleep-on cells (located in the pontine tegmentum), are particularly active during REM sleep, and are probably responsible for its occurrence. The release of certain neurotransmitters, the monoamines (norepinephrine, serotonin and histamine), is completely shut down during REM. This causes REM atonia, a state in which the motor neurons are not stimulated and thus the body's muscles don't move. The lack of REM atonia causes REM Behavior Disorder; sufferers act out the movements occurring in their dreams.

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 that of the surroundings. Erections of the penis or clitoris are also common during REM.

The eye movements associated with REM are generated by the lateral geniculate nucleus and are associated with PGO (pons, geniculate, occipital) waves.

Theories about the function(s) of REM sleep
The function of REM sleep is not well understood; several theories have been advanced.

According to one theory, certain memories are consolidated during REM sleep. Numerous studies have suggested that REM sleep is important for consolidation of procedural and spatial memories. (Slow-wave sleep, part of non-REM sleep, appears to be important for declarative memories). However, in people that have no REM sleep (because of brain damage), memory functions are not measurably affected.

Another theory suggests that 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. Acute REM sleep deprivation can improve certain types of depression, and depression appears to be related to an imbalance of certain neurotransmitters. Most antidepressants selectively inhibit REM sleep due to their effects on monoamines. However, this effect decreases after long-term use.

According to a third theory, known as the Ontogenetic Hypothesis of REM sleep, this sleep phase (also known as Active Sleep in neonates) is particularly important to the developing brain, possibly because it provides the neural stimulation that newborns need to form mature neural connections and for proper nervous system development (Marks et al. 1995). Studies investigating the effects of Active Sleep deprivation have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and result in an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004). REM sleep is necessary for proper central nervous system development (Marks et al. 1995). Further supporting this theory is the fact that the amount of REM sleep decreases with age, as well as the data from other species (see below).

REM sleep in other animals
REM sleep occurs in all mammals and birds. It appears that the amount of REM sleep per night in a species is closely correlated with the developmental stage of newborns. The platypus for example, whose newborns are completely helpless and undeveloped, has 8 hours of REM sleep per night; in dolphins, whose newborns are almost completely functional at birth, almost no REM sleep exists.

History
The association between dreaming and REM sleep was discovered by Eugene Aserinsky and Nathaniel Kleitman in 1953.