REM sleep, short for rapid eye movement sleep, is the stage when most vivid dreaming happens. Far from resting, the brain in REM works roughly as hard as it does when you’re awake.1PubMed. Cerebral O2 metabolism and cerebral blood flow in humans during deep and rapid-eye-movement sleep That busy brain runs alongside a body that is almost completely switched off. REM is less a deeper sleep than a different state, one that seems to do real work on memories and emotions.
Brainwaves in REM Look Almost Like Being Awake
In deep non-REM sleep, brainwaves are big, slow waves of under 4 cycles per second, as huge groups of brain cells fire in unison.2PubMed Central. The role of REM sleep theta activity in emotional memory In REM those waves give way to small, fast, mixed rhythms that closely resemble a calm, awake brain.3PubMed Central. The role of REM sleep theta activity in emotional memory One steady rhythm does stand out: theta, at about 4 to 12 cycles per second, is prominent in REM and links memory and emotion areas.4PubMed Central. Emotional Memory Processing during REM Sleep with Implications for Post-Traumatic Stress Disorder
The recording that shows these brainwaves is called an EEG, short for electroencephalogram. It uses small sensors on the scalp to pick up the brain’s electrical rhythms. Sleep labs pair it with sensors on the eyes and muscles, since REM shows fast brainwaves, darting eyes and limp muscles all at once. That combination is how REM gets told apart from waking, even though the brainwaves alone look similar. Some scientists call REM paradoxical sleep because the brain looks awake, yet the person is hard to rouse and their body is limp.
Sleep runs in cycles of roughly 90 minutes, with REM following the deeper stages. REM periods grow longer toward morning, so most REM comes in the last part of the night. Cutting your sleep short by an hour or two therefore trims REM disproportionately.
Your Brain Burns as Much Energy in REM as When Awake
The brain’s oxygen use during REM is practically the same as when you’re awake.5PubMed. Cerebral O2 metabolism and cerebral blood flow in humans during deep and rapid-eye-movement sleep Blood flow to the brain rises and falls in step with that energy use.6PubMed. Cerebral O2 metabolism and cerebral blood flow in humans during deep and rapid-eye-movement sleep Across many brain areas, blood flow, nerve-cell firing, metabolism and temperature all climb during REM.7PubMed. A brain-warming function for REM sleep One idea is that REM periodically rewarms a brain that cools during deep sleep.8PubMed. A brain-warming function for REM sleep That warming role is a proposal, not a settled fact.
Deep sleep is the brain’s lower-energy phase, with slower firing and reduced blood flow. REM switches it back to a high-energy state, and this swing repeats several times a night. A brain this busy may help explain why dreams can feel as rich as waking life.
Emotional and Visual Areas Light Up While Planning Areas Quiet
Brain scans show that REM especially boosts activity in old, deep regions tied to emotion, including the amygdala, the brain’s alarm center.9PubMed. Forebrain activation in REM sleep: an FDG PET study Areas that handle reward, bodily feelings and memory become more active too.10PubMed. Forebrain activation in REM sleep: an FDG PET study
The visual side has its own driver. During REM, bursts of electrical signals travel from the brainstem to the brain’s visual relay station and the vision area at the back of the head.11PubMed Central. The Ponto-Geniculo-Occipital (PGO) Waves in Dreaming: An Overview The visual brain receives input with no real image behind it, which it may treat as seeing, producing dream visuals.12PubMed Central. The Ponto-Geniculo-Occipital (PGO) Waves in Dreaming: An Overview
Scans also show some regions involved in planning and self-checking running relatively quieter, a pattern that fits how dreams feel. With those checking systems turned down, you accept flying or sudden changes of place without questioning them. This is a common interpretation of the scan findings, not a proven explanation of dreaming.
The same brainstem bursts that feed vision occur around the darting eye movements that give REM its name. Those movements aren’t literally watching dream scenes in any proven way. Sleep labs use them as a key marker of REM.
Why You Can’t Move During Dreams
During REM, cells in the brainstem recruit nerve cells that shut down the nerves driving your skeletal muscles.13PubMed Central. REM Sleep at its Core – Circuits, Neurotransmitters, and Pathophysiology They do it by releasing two calming messenger chemicals, GABA and glycine, that hold those muscle nerves switched off.14PubMed Central. Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis Both are needed: blocking both in animals stops the paralysis.15PubMed Central. REM Sleep at its Core – Circuits, Neurotransmitters, and Pathophysiology And when mice lost inhibitory nerve cells in part of the spinal cord, they twitched and jerked during REM.16PubMed Central. Brainstem and Spinal Cord Circuitry Regulating REM Sleep and Muscle Atonia The paralysis is an active brake, not muscles simply relaxing.
The brake is selective. Breathing muscles keep working, as does the heart. The eye muscles are spared, which is the source of “rapid eye movement.” Small twitches of fingers, toes or the face still slip through, and these are normal, so small twitches alone are not evidence of a problem.
Sometimes you wake while the brake is still on, leaving you briefly unable to move. This is sleep paralysis. It can come with a sense of a presence or dream imagery, because dreaming hasn’t fully stopped. It usually passes within seconds to a couple of minutes and is generally harmless. It is more likely with sleep deprivation and irregular sleep schedules.
The Brainstem Switch That Turns REM On and Off
The brainstem holds a REM-on area and a REM-off area that each suppress the other.17PubMed. A putative flip-flop switch for control of REM sleep When one side wins, it strongly silences the other, making the shift into or out of REM quick and clean.18PubMed. A putative flip-flop switch for control of REM sleep When one side gains strength, it suppresses its rival even more, which removes the rival’s opposition. That feedback produces sharp transitions instead of a slow fade.
The REM-on side sends one set of signals up to produce awake-like brainwaves and another down to the spinal cord to paralyze muscles.19PubMed. A putative flip-flop switch for control of REM sleep A single switch driving both effects explains why the fast brainwaves and the limp body arrive together. Meanwhile, wake-promoting brain cells that use a chemical called orexin (also known as hypocretin) push toward the REM-off side, helping keep REM from intruding while you’re awake.20PubMed Central. Sleep State Switching
A switch built this way can flip at the wrong moment, as in narcolepsy.21PubMed. A putative flip-flop switch for control of REM sleep People with narcolepsy can drop into REM within minutes of falling asleep. In many people, strong emotions like laughter can trigger sudden muscle weakness, a piece of REM paralysis while awake. Vivid dream images at sleep onset and sleep paralysis are common. The most common form of narcolepsy involves loss of orexin cells, which fits the switch picture.
How REM Sorts Memories and Softens Emotions
After a night that includes REM, the amygdala reacts less to upsetting images seen the day before, and people rate them as less upsetting.22PubMed Central. REM sleep de-potentiates amygdala activity to previous emotional experiences One reason may be that levels of noradrenaline, a stress chemical, drop sharply during REM.23PubMed Central. REM sleep de-potentiates amygdala activity to previous emotional experiences With that chemical low, the brain can replay emotional memories without the full alarm response attached. REM may let you keep the facts of an experience while dialing down its emotional charge.24PubMed Central. Sleep and Emotional Memory Processing Computer models add that theta rhythms in REM strengthen a planning area’s control over the amygdala, quieting fear responses.25PubMed Central. Emotional Memory Processing during REM Sleep with Implications for Post-Traumatic Stress Disorder
Cutting a night short, especially by waking early or going to bed very late, reduces REM and may leave reactions sharper the next day. Much of the evidence comes from labs and animals, so this describes a likely role of REM, not a guarantee that sleep fixes distress.
REM also edits connections. In mice learning a new movement task, REM removed many newly formed connections between brain cells.26PubMed Central. REM sleep selectively prunes and maintains new synapses in development and learning It also strengthened a select few that mattered for improving at the task.27PubMed Central. REM sleep selectively prunes and maintains new synapses in development and learning REM may also split into busier bursts with eye movements and quieter stretches, which may help with different kinds of learning.28PubMed. The microstructure of REM sleep: Why phasic and tonic? Sleeping after practice, not just practicing more, supports learning, and short nights after learning something new may blunt the overnight gain.
When REM Brain Activity Goes Wrong
In REM sleep behavior disorder, or RBD, the muscle brake fails and people shout, punch or kick out their dreams.29PubMed Central. REM Sleep Behavior Disorder as a Prodromal Synucleinopathy: Updates on Clinical and Laboratory Biomarkers, and Implications for Neuroprotective Trials It’s diagnosed by an overnight sleep study with video that shows muscles active during REM.30PubMed Central. REM Sleep Behavior Disorder as a Prodromal Synucleinopathy: Updates on Clinical and Laboratory Biomarkers, and Implications for Neuroprotective Trials
RBD can be an early sign of brain conditions such as Parkinson’s disease, sometimes years ahead.31PubMed Central. REM Sleep Behavior Disorder as a Prodromal Synucleinopathy: Updates on Clinical and Laboratory Biomarkers, and Implications for Neuroprotective Trials Anyone who repeatedly acts out dreams should see a doctor or sleep specialist.
RBD differs from sleepwalking. Sleepwalking comes from deep non-REM sleep, often early in the night, and the person usually recalls little or no dream. RBD tends to occur later, when REM is more common, and the person often recalls a matching dream, frequently of being chased or attacked. A bed partner’s description of shouting, punching or flailing is often the first clue. Because it can cause injury, simple safety steps such as moving sharp furniture away from the bed help while waiting for evaluation.
In PTSD, stress chemical levels appear to stay high during REM.32Journal of Neuroscience. Neuroscientists learn why PTSD patients relive highly charged fear memories in sleep The rhythms that normally quiet fear memories may then no longer work, letting fear memories replay.33Journal of Neuroscience. Neuroscientists learn why PTSD patients relive highly charged fear memories in sleep That fits the repeated, vivid nightmares many people with PTSD describe.
Why Babies Spend So Much Time in REM
Newborns spend a very high share of sleep in REM, a share that drops quickly as they grow.34PubMed Central. Sleep as a driver of pre- and postnatal brain development REM may help sensory and movement areas mature and prepare for the flood of input after birth.35PubMed Central. Sleep as a driver of pre- and postnatal brain development In the developing brain it also prunes new connections and strengthens others, much as it does in learning.36PubMed. Rapid eye movement sleep and neuronal development
Parents see this as “active sleep,” the infant form of REM. Newborns twitch, flutter their eyelids and breathe irregularly. This is normal and not a sign of distress. Babies often enter active sleep soon after falling asleep, unlike adults.
Adults spend roughly a fifth to a quarter of sleep in REM, and that share tends to dip somewhat in older age. Alcohol and some antidepressants suppress REM, which can bring vivid rebound dreams when they’re stopped. REM-like brain activity has also been reported in reptiles, fish, flies, worms and octopus relatives.37PubMed Central. Non-REM and REM/Paradoxical sleep dynamics across phylogeny