Discovered in 1953, phases of sleep during which the eyes move rapidly (although the eyelids are closed). Also known as REM, these phases occur at regular intervals during sleep (there is a cycle which lasts about 90 minutes).

Scientists have discovered many interesting facts about REM, including:

  • Brain activity during REM starts in the part of the brain called the pons.
  • Research subjects who are awakened during REM sleep can almost always remember their dreams.
  • The body cannot move during REM sleep.
I don't have a reference for it, but I have heard that if you deprive someone of REM sleep, they will go bonkers.

refs: the QPB Science Encyclopedia, and

An elaboration on the interesting facts about REM sleep in the write-up above:

- In sleep studies in cat brains, the definitive brain event of REM sleep is a series of synchronized firings called pontine-geniculate-occipital waves, or PGO spikes. The neurons that start the waves are located in the pons, (around the locus coeruleus,) and synapse on the neurons of the lateral geniculate bodies of the thalamus. From there, the waves are propagated to the occipital cortex. Why is this neat? The lateral geniculate and occipital cortex are parts of the visual system in the brain. Not only that, but the PGO waves are also propagated in to the limbic cortex, an area of the brain responsible for emotional responses and memory among other things. The PGO waves are essentially tickling the emotional and visual parts of the brain, and this non-specific stimulation is considered to be the substrate of the emotional and visual nature of dreaming. In other words, it is probable that PGO waves make dreams what they are.

- Not only are you paralyzed during REM, but your body also fails to carry out thermoregulation. Although we don't exactly know why we sleep (see Sleep: the metanode, why do we need to sleep?, the teleological argument for sleep, and sleep and memory for some perspectives,) it is most likely that it has something to do with the evolutionary advantages of body energy conservation and thermoregulation. However, during REM, your body cannot regulate its temperature. Since infants spend nearly 25% of their early life in REM sleep, this failure to thermoregulate may be one cause of SIDS. What could the evolutionary advantage of REM sleep be, if it puts the body in danger?

- Some neuroscientists have theorized that REM sleep is directly related to memory encoding and storage - and this makes sense considering that the hippocampus is also stimulated by PGO waves, and the hippocampus has been associated with memory functions since the famous case of HM. While studies of selective sleep deprivation (i.e. only REM, not non-REM deprivation) have not shown anything that confirms this, recent work by Matthew Wilson, a neuroscience researcher at MIT has done some work showing that lab rats dream about the mazes they run. Thus, REM sleep might be advantageous as a mechanism for memory consolidation.

More about REM-sleep

-Amount of REM-sleep positively correlates with IQ among children, that is: Intelligent children have more REM than usual.

-Selective REM-sleep deprivation has a similar effect to antidepressants such as Prozac on depression; a lasting positive effect that takes a few weeks to develop. And also, it seems that ALL medications that have an antidepressant effect causes some degree of selective REM sleep deprivation.

-In addition to the memory/consolidation hypothesis of REM sleep, there is the brain development hypothesis. REM sleep serves development of the brain. The hypothesis is supported by these facts:
--The period in our life that we spend most time in REM sleep is the same period in which our brain development is at its peak.
--The amount of REM sleep decreases throughout life from 70% of total sleep time as newborns, to 15% in late adulthood.
--New-borns of species that are born with well-developed brains (such as the guinea pig), spend proportionally less time in REM sleep than new-borns of species where the brain development is very active after birth (e.g. humans)

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