REM was discovered in the 1950s and in the next few decades scientists tried to figure out why Nature made REM and what it does for us. The EEG readings of a person in REM looks like that of a waking person, and the consciousness experienced in REM (often narratively coherent dreams) is more like waking than like the comparative subjective darkness of NREM sleep.
Also called “paradoxical sleep”, REM is almost a hybrid of waking the NREM sleep. Some neurons and brain areas are as active as during waking; others are silent or dormant. Even deep in the brainstem some cells (e.g. cholinergic neurons) fire away during REM while some (e.g. monoaminergic neurons) stop firing. Drug stimulation of the cholinergic neurons can increase time spent in REM, but scientists know there is a lot more to REM than simple switching of neuron firing, even if they can’t totally explain everything.
Memories and REM
One idea about REM was that it was important in the consolidation of memories. This makes sense because the subject of dreams in REM often involve or borrow from the previous day’s events.
Further, we spend less time in REM the older we get. Do young children spend more time in REM than old people because they are learning so much? It seems plausible that there would be a connection.
Research in recent decades has found memory consolidation and translation from short-term to long-term memory occurs more in NREM sleep. Learning facts and how to do things – these are facilitated by NREM and the memories are enforced and hardened.
During REM the emotional center of the brain, the amygdala, is very active. This may explain the strong feelings that can be associated with the dreams we remember in REM, and may help connect emotions to memories consolidated in NREM. A scenario or incident transferred to long-term memory in NREM may be replayed, perhaps with a fantastic twist, during REM, and emotion associated with it.
Further, there is no correlation between time spent in REM and IQ or other measures of cognitive capability.
Development of the brain
Another idea is that in some way REM is important in the growth and development of the person’s body, and in particular the nervous system. Newborn babies spend 8 hours per day in REM and even fetuses experience REM. Small children spend more time in total sleep and in REM than adults do. While the current thought is that REM may partially function to assist neurological development, it did not evolve for that reason and it stands separately as a form of consciousness, not as a way to help the brain grow.
Emotion is tied up with sleep and tired brains, and people are generally more upbeat in the morning after a good night’s sleep. But REM does not seem to make us optimistic or particularly happy. Indeed, REM is associated with depression. Anti-depressant drugs (the popular SSRI type) tend to suppress REM sleep and it turns out this isn’t just a side effect, but appears tied in to how they reduce depression.
People with post-traumatic stress disorder spend more time in REM than other people. Indeed, an increasingly accepted counter-measure to inhibit PTSD development is to keep the victim from going to sleep after the stressful incident. People with difficult or traumatic pasts often have psychiatric problems that are exacerbated by dreaming about them. Replaying traumatic incidents over and over during REM dreams makes waking life worse.
Scientist J. Allan Hobson published a theory that REM is a sort of virtual reality program run by the brain. In this view REM is a “protoconscious state” that helps us rehearse and mentally act out scenarios in a manner similar to waking playtime does. This is how it helps us function in the world when we are awake and this is why it evolved.
Play is well-known to assist in a number of cognitive benefits, http://udel.edu/~roberta/play/benefits.html
While this idea has not been universally accepted, the absence of psychological explanations for dreaming and REM is pretty much widely agreed to by serious scientists.
Thermoregulation and REM Evolution
Because warm-blooded animals experience REM, it is thought that REM arose in evolution around the same time as thermal homeostasis. Brain temperature drops during NREM but rises in REM. The thermal regulation of the body during sleep differs from waking. You don’t sweat or shiver during REM sleep but your body uses other mechanisms to affect internal temperature. The normal thermoregulation homeostasis is off-line, so to speak.
One theory holds that at one point our ancestors were nocturnal, sleeping in the daytime. During twilight the ambient temperature was about the same as the internal body temperature, so contraction of muscles was not needed. In this theory primitive sleep that happened during the daytime evolved into NREM sleep, while sleep that happened at twilight evolved into REM sleep. This may be partly why the EEG signature of the brain in REM looks like that of a waking brain – REM is a warm-up (thermally and electrically) for waking. This theory also provides a partial explanation for long-term memory formation. During NREM “uncoordinated reinforcement” of memories fire in the neural circuit, while in REM a coordinated stimulation, perhaps in the virtual reality of the mind, shape the memories into a coherent whole that the waking mind understands. The one-two punch of NREM followed by REM sleep is what makes the mind form long-term memories.
Neuroscientist W.R. Klemm postulates that REM helps the brain wake up. This certainly meshes with the evidence from EEG readings that show REM and waking to be similar and with the subjective reports of people woken during REM who describe their dreams as much more vivid and coherent than those woken during NREM sleep. The brain literally gets warmer during REM and activity in the cortex increases as if to imply a revving up in anticipation of waking. In his Wake-Up Hypthoseis Klemm points out that REM and waking both rely on similar brainstem arousal systems.
The periods of REM during a night therefore act partially as a test period in which the brain evaluates whether enough sleep has been experienced yet. This gives an explanation, if not a convincing one, of why REM periods increase in duration and frequency throughout the night and why sleepers go through a long REM cycle shortly before awakening.
REM is in a way a dress-rehearsal for waking life in this theory. This also explains why young children have more REM: to develop their cognitive capabilities they need additional practice and mental stimulation. Klemm points out that waking is the most biologically adaptive form of consciousness, so we have an advantage in leaving NREM as soon as our brains are rested and NREM needs have been satisfied.
Jerome Siegel at UCLA says REM has little or no physiologically critical role and cannot be considered as important as slow-wave sleep. He told the New York Times that individuals who cannot experience REM do not have cognitive or emotional handicaps. Siegel speculates that REM functions to prepare the brain for waking. The body is allowed to continue to rest but the brain and mind starts to hum. Steven Lima at Indiana State told the Times he hypothesizes REM is a way to let part of the brain sleep while other parts wake. Dividing up the period of inactivity and reduced awareness this way may have allowed our ancestors to better survive in a predator-rich environment.
Another advantage to the flip between REM and NREM during the night is that is allows different parts of the brain to rest at different times.
REM Sleep – Stage R in the Sleep Cycle
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.
Dreaming During Stage R
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.
Neurotransmitters and 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.
Behavior During Stage R Sleep
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.