Called Rapid Eye Movement because of characteristic twitching of the eyelids and surrounding muscles, REM sleep is the weirdest brain state most people experience on a regular basis. If sleep is mysterious, REM sleep is even more mysterious. REM phenonmena has wormed its way into folklore over the centuries because the brain is active and the skeletal muscles have no tone.
The muscles associated with breathing are not paralyzed, but breathing in REM is shallower and more rapid. Compared to NREM, the body has higher blood pressure and heart rate in REM. EEG readings show both alpha and beta waves. The pattern is more like waking than it is like NREM sleep. In a polysomnogram it is easier to tell a person is in REM sleep by looking at the electromyogram (muscle activity) and electrooculogram (eye movement). Some see REM as a hybrid state between sleep and waking, although that is incorrect and reflects a misunderstanding of what sleep actually is. But people think that because REM looks like waking on an EEG and the cerebral cortex is quite active.
Sleepers in the REM stage appear immobile except for their breathing and fluttering eyes. The large skeletal muscles are paralyzed (atonia). An area called the pons in the base of the brain signals the spinal cord to shut down neurons. This is thought to be evolution's solution to the brain's playing out of stories in dreams. if the sleeper could move around, he or she would play out dreams and perhaps endanger themselves or others. Indeed, a rare disorder called REM sleep behavior disorder occurs when the skeletal muscles are NOT paralyzed during REM. It can be dangerous.
Looking deeper into REM sleep as it appears to an external observer, it has been proposed to divide the period into "tonic" and "phasic" periods. There is no muscle tone (atonia) in the skeletal muscles during the tonic period while in the phasic period the body has muscle twitches and bursts of rapid eye movements.
During REM we are "vigilant" and it is pretty weird. A loud sound might not wake you up, but a whispered familiar name might. Emotions rule; the limbic and paralimbic systems seem to be more active during REM. The brain is excited in some ways as it is during waking but muscles are immobile. "Paradoxical sleep" is another name for REM although rarely used any more. The professional society American Academy for Sleep Medicine has proposed using the term "Stage R" for REM sleep, but this term is rarely used in either academic or popular literature.
In normal sleep architecture, nightly sleep starts with NREM (non-REM) sleep and episodes of REM intrude on the NREM sleep. The first REM period happens 70 to 90 minutes after sleep onset. At first these REM episodes are short (a few minutes), but as the night progresses, they become longer. Most REM sleep is late in the main sleep period (close to morning). A typical adult spends about an hour and a half every night in REM.
Newborns spend 8 hours a day in REM, perhaps because they are learning so much about the world. As the child grows, the duration of REM declines until it reaches adult levels in the middle teens.
The body becomes poikilothermic during REM - which means the normal temperature regulation goes awry. The body temperature is not controlled.
REM (rapid eye movement) sleep behavior (paralyzed skeletal muscles and twitching eyelids) had been observed for centuries, and was first noted in the scientific literature in the 1930s. It was known that if you wake up a person with twitching eyelids he would often (but not always) report being in the middle of a dream. In the 1950s scientists established rapid eye movement as a stage of sleep. It is sometimes called the 5th stage (or 4th stage under the new classification.) It was first thought, and is still widely believed by the lay public, that REM is synonymous with the dream state. That's not true; we now know dreams can occur anytime in the sleep cycle, but the most vivid dreams tend to happen in REM.
Release of some neurotransmitters by brain cells stops during Stage R, and this halting gives some insight into skeletal muscle paralysis during this period. Cells that make norepinephrine, serotonin, and histamine (all monoamines) stop releasing them during REM. Constant release of monoamines may desensitize receptors. By stopping the release, the body allows the receptors to reset and regain sensitivity. This might be a partial reason why lack of sleep makes you cranky. We know serotonin has a connection with mood.
Scientists have found that when fruit flies are in an inactive period, their monoamine levels decrease. This suggests REM derives from a very ancient evolutionary part of the sleep cycle.
The neurotransmitter acetylcholine is intimately caught up with REM. Neurons release the most acetylcholine during REM and waking, and the least during slow-wave sleep. Drugs that antagonize acetylcholine result in less REM sleep. Scientists don’t know how the muscle paralysis mechanism works during sleep, but they do know the neurotransmitters glycine and GABA are both important.
When researchers activated acetylcholine-releasing neurons in mice, they found they were able to trigger a REM period. This is exciting because medicines are not able to advance a brain in the sleep cycle. Further, promoting REM by this manner allows recreation of a more-or-less natural sleep cycle. One disadvantage of medicines for sleep is that they only sedate the patient rather than promoting any stage of sleep or the sleep stage cycle. Of course, direct stimulation is impractical as a therapy for humans, but this finding could advance a broader understanding of the sleeping brain.
The regulation of REM is also of some question. While sleep is under some homeostatic control (one of the processes of the 2-process model), it is less clear that REM is. When a person is deprived of REM, a REM sleep debt builds up and extra REM occurs in subsequent sleep – although not all that is lost is recovered. However, there is no indication that this recovery REM is more intense than regular REM. This is in contrast to NREM sleep, which is measurably more intense in a person who has been sleep deprived.
If we have stages of NREM sleep, why not stages of REM? It appears that REM is much more homogeneous than NREM. Stages 2 and 3 of NREM produce completely different EEG patterns, but REM’s EEG does not change much. Researchers (but not medical practitioners) sometimes distinguish phasic and tonic REM based on whether the sympathetic or parasympathetic nervous systems appear to be controlling. The phasic component, influenced by the sympathetic nervous system, results in eye movements, some twitching of the muscles, and variable breathing rates and depths. The tonic component, under the parasympathetic nervous system, does not result in eye movements – the person is more quiet. Tonic REM is persistent and is interrupted by outbursts of phasic activity.
All mammals experience REM except dolphins and seals, so it is wrong to think of REM as a hallmark of a particularly advanced brain. Even dumb animals like horses go through REM. First discovered in humans in 1953, REM was soon after discovered in animals. REM stage sleep EEG doesn’t look too much different from waking patterns, which fits with the two-phase theory of sleep’s holding that REM is like being awake and asleep at the same time.
The connection between learning and REM grows stronger as scientists learn more. It explains why infants spend such a high proportion of their sleep in REM - infants have so much to learn. When sleep is disrupted, the brain's ability to transfer short-term memory into long-term memory is impaired.
Sleepers in REM do not respond readily to external stimuli. They are more like people in deep sleep than people in Stage 2 sleep. Even unborn fetuses have REM. However, spontaneous awakening from REM is common. Sleep-disordered breathing during REM more fragmented than normal, so although those with apnea may end up the night with the same quantity of time spent in REM, it is spread over more periods. Whether this is a negative or not is unclear.
The U.S. government's Board on Health Sciences Policy published Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem (2006) which includes a handy table showing some differences between REM sleep and Non-REM sleep
|Body temperature||Homeostatic set point lower than when awake||Not regulated. No sweating or shivering.|
|Respiration||Lower than when awake||More than Non-REM. Coughing may be suppressed.|
|Brain activity||Lower than when awake||Motor and sensory areas have more activity than during Non-REM|
|Blood pressure||Lower than when awake||Higher than in Non-REM|
|Heart rate||Slower than when awake||Higher than in Non-REM|
|Muscle tone||Same as when awake||None in major skeletal muscles|
|Sympathetic nervous system activity||Lower than when awake||Higher than when awake|
There are anti-depressant drugs that suppress REM and barbiturates do, too. This is considered an undesirable side effect of these drugs, and if the modification to the sleep architecture is severe, can be reason for stopping use of the drug.
People are more alert when awakened from REM than from deep sleep. Upsets in the sleep cycle that result in lost Stage R sleep lead to a "REM sleep debt" and subsequent REM sleep rebound. Deprivation from REM does not cause insanity (as was once thought), but can cause irritability. Too much REM, on the other hand, is associated with depression. Waking up in the middle of REM often results in a dream that can be recalled. And waking during REM tends to result in a negative self-image and self-appraisal.