What Is Rem Sleep?
REM is an acronym for rapid eye movement, and it is named this way because when someone is in rem sleep their eyes will be moving rapidly around the motion is random, this contrasts with the other stages of sleep where the eye stays static. REM sleep is further classified into two categories, tonic REM sleep and phasic REM sleep. As well as the eye movement there are other physiological signs that REM is occurring, this includes a rapid, low voltage EEG and low muscle tone. All land mammals have REM sleep.
In humans REM sleep takes up from a fifth to a quarter of the total amount slept. It is broken up into small chunks and tends to occur between 4 and 5 times a night. The length of the REM sleep tends to be short in the early part of the night and lengthen as sleep goes on. The relative amount of REM sleep vs non REM sleep tends to vary with age also, with a newborn baby spending over 80% of total sleep time in REM. Another characteristic of REM sleep is that brain's neurons seem to be behaving in a similar manner as to when someone is awake, this gives rise to the nick name of REM sleep as the paradoxical sleep. The dreams that are most vividly remembered tend to occur in the REM part of sleep. REM is thought to be a very important part of the sleep cycle, if rem sleep is repeatedly interrupted then organisms will compensate for it by having a rebound sleep with more REM sleep.
Theories about the function of REM sleep
There are various theories as to the function of REM sleep.
Memory related
It is thought that memories are consolidated during REM sleep, this is backed up by many studied that have indicated that REM sleep is a very important factor for both spatial and procedural memory. One study indicated that improving REM sleep improves the recall of memorized pairs of words. Interestingly it is was shown that a daytime nap which had no REM sleep helped declarative memory but not procedural memory. It should be noted though that this theory is not water tight as for example Monoamine oxidase (MAO) inhibitors and tricyclic antidepressants suppress REM sleep, but these drugs do not impair memory, indeed some studies even suggest that MAO inhibitors improve memory.
Developmental
Another theory posits that this sleep stage is important for development. It is known as the Ontogenetic Hypothesis of REM sleep. This theory is given more weight by the fact that newborns tend to be in REM for so long. Potentially because it provides the neural stimulation which newborns require to form the connections in their brain and nervous system. Some studies which weer looking into the effects of sleep deprivation have indicated that sleep deprivation early in someones life may lead to problems with behaviour, as well as permanent sleep disruption, decreased brain mass. From this theory leads the thought that for a mature brain REM sleep is not that important, however on further thought actually it would still be important as even in a mature brain the neurons are constantly changing.
Monoamine recovery
This theory posits that REM sleep is there to shutdown monoamine, this can then allow the monoamine receptors within the brain to recover.
Sentinel theory
This theory says that REM sleep is there so that an animal can periodically come into a wakeful state so that it can scan for danger. It was put forward by Frederick Snyder in 1966 and is based on the observation that the REM sleep in some mammals is followed by a very brief awake period.
Summary
REM sleep is a very important part of sleep, if it is interrupted or reduced in any way then there will tend to be a rebound effect and more REM sleep will occur. Indeed prolonged REM sleep deprivation has lead to death in experimental animals.