REM vs Deep Sleep: What Each Stage Does and Why Both Matter
Every morning, your sleep tracker delivers a breakdown of last night's sleep stages — light, deep, REM — accompanied by percentages that imply a clear hierarchy. Deep sleep good. REM sleep good. Light sleep... filler? The reality is more interesting and more consequential than the dashboard suggests. Deep sleep and REM sleep serve fundamentally different biological functions, they are regulated by different neurological mechanisms, and they respond to different environmental and behavioral factors. Understanding what each stage actually does is the first step toward meaningfully improving either one.
What Happens During Deep Sleep (N3)
Deep sleep — also called slow-wave sleep or N3 in the clinical staging system — is defined by the presence of high-amplitude, low-frequency delta waves (0.5 to 4 Hz) on an electroencephalogram (EEG). During this stage, brain activity slows dramatically compared to wakefulness. Heart rate drops by 20 to 30% from its waking baseline. Respiratory rate decreases and becomes highly regular. Core body temperature reaches its nightly nadir. The arousal threshold — the amount of stimulation needed to wake the sleeper — is at its highest, making deep sleep the most difficult stage from which to be awakened.
The primary functions of deep sleep are physical restoration and declarative memory consolidation. Growth hormone release peaks during the first deep sleep cycle of the night — approximately 70% of the daily growth hormone output occurs during N3, according to research from the University of Chicago. This hormone drives tissue repair, muscle growth, and immune cell production. A single night of deep sleep deprivation has been shown to reduce growth hormone secretion by 70%, with downstream effects on next-day inflammatory markers and wound healing speed.
Deep sleep is also the stage during which the glymphatic system — a waste-clearance network discovered by Maiken Nedergaard at the University of Rochester in 2013 — is most active. During N3, the interstitial spaces in the brain expand by approximately 60%, allowing cerebrospinal fluid to flush metabolic waste products, including beta-amyloid, the protein associated with Alzheimer's disease. A 2019 study in Science found that slow-wave electrical activity during deep sleep preceded and predicted each pulse of cerebrospinal fluid flow through the brain — the waves literally drive the cleaning cycle.
Declarative memory — facts, events, and learned information — is consolidated during deep sleep through a process called memory replay. The hippocampus, which stores recent memories temporarily, replays neural patterns from the day's learning during N3 at an accelerated rate, transferring information to the neocortex for long-term storage. A study by Jan Born's group at the University of Tübingen demonstrated that stimulating slow-wave oscillations with transcranial direct current during sleep improved next-day recall of word pairs by 8% compared to sham stimulation.
What Happens During REM Sleep
REM sleep — rapid eye movement sleep — is neurologically closer to wakefulness than to any other sleep stage. Brain activity during REM, as measured by EEG, is nearly indistinguishable from waking brain activity: high-frequency, low-amplitude waves dominate, and metabolic activity in many cortical regions equals or exceeds waking levels. The defining features of REM are the rapid, saccadic eye movements visible through closed lids and the near-complete paralysis of skeletal muscles (atonia), which prevents the sleeper from physically acting out dreams.
REM sleep serves distinct functions from deep sleep, centered on emotional regulation and procedural memory consolidation. During REM, the prefrontal cortex — the brain region responsible for rational, executive control — is less active, while the amygdala — the emotional processing center — is highly active. This creates a state in which emotional memories are reprocessed in a context stripped of executive inhibition. Dr. Matthew Walker has described REM sleep as "overnight therapy" — the brain revisits emotionally charged experiences from the day and strips them of their acute emotional intensity while preserving the informational content.
A 2011 study by Walker and colleagues in Current Biology provided direct evidence for this mechanism. Participants viewed emotionally provocative images, then either slept (with polysomnography) or stayed awake for 12 hours before re-evaluating the images. Those who slept showed reduced amygdala reactivity to the images — they remembered the content but the emotional sting had diminished. Those who stayed awake showed no reduction. Critically, the degree of emotional reset correlated specifically with the amount of REM sleep obtained, not total sleep time.
Procedural memory — motor skills, spatial navigation, and pattern recognition — is consolidated during REM rather than during deep sleep. A study in Nature Neuroscience found that participants learning a complex finger-tapping sequence showed significant improvement after a night of sleep, and the improvement correlated with the amount of REM sleep during the final third of the night (when REM is most abundant). Disrupting REM selectively — while leaving deep sleep intact — eliminated the overnight improvement.
How the Two Stages Are Distributed
Deep sleep and REM sleep are not evenly distributed across the night. They follow an architectural pattern that most sleep trackers visualize as a hypnogram — the staircase-like graph showing sleep stages over time. Understanding this pattern is important because the timing of your sleep — not just the duration — determines how much of each stage you get.
Deep sleep is concentrated in the first third of the night. The first sleep cycle (approximately 90 minutes long) contains the largest block of N3 — typically 20 to 40 minutes. The second cycle contains a shorter block. By the third cycle, deep sleep often diminishes to less than 10 minutes, and by the fourth and fifth cycles it may disappear entirely.
REM sleep follows the opposite pattern. The first REM period, which closes the first 90-minute cycle, is typically short — 5 to 10 minutes. Each subsequent REM period grows longer, with the final cycles of the night containing REM periods of 30 to 60 minutes. By the last third of the night, REM dominates and deep sleep is largely absent.
This front-loading of deep sleep and back-loading of REM has a critical practical implication: if you cut your sleep short by waking up early, you disproportionately lose REM sleep. If you go to bed late but wake at a fixed time, you disproportionately lose deep sleep (because the first, deepest N3 blocks are pushed into a compressed early-night window). Neither truncation is harmless, and each produces different deficits.
What Low Deep Sleep Means
If your sleep tracker consistently shows low deep sleep percentage (below 13% of total sleep time for adults under 60), several factors may be contributing. Alcohol consumption is one of the most common culprits — even moderate amounts suppress deep sleep by 20 to 40% on the same night. Late-evening caffeine (within 6 hours of bedtime) also reduces deep sleep, as caffeine blocks adenosine receptors that promote sleep pressure.
Age is a non-modifiable factor. Deep sleep declines naturally with age — a 20-year-old typically spends 15 to 20% of total sleep time in N3, while a 60-year-old may spend only 5 to 10%. This decline is driven by structural changes in the prefrontal cortex that reduce the brain's ability to generate the slow-wave oscillations that define N3. It is one of the reasons older adults report less restorative sleep even when total sleep time is adequate.
Physical exercise reliably increases deep sleep. A meta-analysis in Sleep Medicine Reviews found that regular moderate aerobic exercise increased deep sleep by 12 to 15% compared to sedentary controls. The effect was most pronounced when exercise occurred in the morning or early afternoon — late-evening vigorous exercise, which raises core body temperature, can actually reduce deep sleep if it occurs within 2 hours of bedtime.
What Low REM Sleep Means
Low REM percentage (below 18% of total sleep time) is often a signal of one of three things: sleep truncation, alcohol use, or antidepressant medication. REM is concentrated in the final sleep cycles, so anyone who chronically undersleeps by even 30 to 60 minutes loses a disproportionate amount of REM. The simplest fix is extending sleep by going to bed earlier rather than waking later (to preserve the morning REM-dominant cycles without disrupting the next day's schedule).
Selective serotonin reuptake inhibitors (SSRIs) — the most commonly prescribed class of antidepressants — potently suppress REM sleep. A 2017 review in Journal of Clinical Psychopharmacology found that SSRIs reduced REM sleep by 25 to 50% during active treatment. This is a known side effect, and the clinical significance is debated — some researchers have suggested that REM suppression may actually contribute to the antidepressant effect, as REM sleep amplifies emotional memories while REM deprivation blunts emotional reactivity. If you are on SSRIs and concerned about low REM, discuss the tradeoff with your prescribing physician rather than adjusting medication independently.
Stress and anxiety can either increase or decrease REM sleep depending on chronicity. Acute stress often increases REM — the brain has more emotional material to process. Chronic, sustained stress can reduce REM by fragmenting sleep architecture and increasing overnight cortisol, which suppresses REM progression. Addressing the underlying stressor through therapy, lifestyle changes, or structured relaxation practices is more effective than any sleep-specific intervention for normalizing REM in this scenario.
Age-Related Changes in Sleep Architecture
The ratio of REM to deep sleep shifts substantially across the lifespan, and understanding these changes helps set realistic expectations for what healthy sleep looks like at different ages. Newborns spend approximately 50 percent of their total sleep time in REM — a proportion that supports the rapid neural development occurring during the first year of life. By age five, REM drops to roughly 25 percent, where it remains relatively stable through adulthood. Deep sleep follows a different trajectory: it peaks in childhood and adolescence at 20 to 25 percent of total sleep time, then declines steadily, reaching 5 to 10 percent by age 60 and sometimes disappearing almost entirely in adults over 75.
This age-related decline in deep sleep is not simply a consequence of aging — it appears to be both a cause and a result of age-related cognitive decline. Research from the University of California, Berkeley has shown that the slow oscillations generated during deep sleep are critical for transferring memories from the hippocampus to the prefrontal cortex for long-term storage. As deep sleep diminishes, this memory consolidation process becomes less efficient, contributing to the forgetfulness commonly attributed to aging. Interventions that enhance deep sleep in older adults — including acoustic stimulation timed to slow-wave oscillations — have shown promise in improving next-day memory performance by 20 to 40 percent in preliminary studies.
The practical implication is that sleep improvement strategies should target the sleep stage most likely to be deficient for your age group. Adults under 40 who report poor sleep quality are more likely to be REM-deprived, often due to alarm-clock interruption of the final REM period (which typically occurs in the last 60 to 90 minutes of sleep). Adults over 50 are more likely to benefit from strategies that enhance deep sleep: maintaining a cool bedroom (65°F or below), exercising earlier in the day, and avoiding alcohol, which suppresses deep sleep even in moderate quantities. A sleep tracker that reports stage percentages can help identify which type of sleep you are consistently missing.
Age-Related Changes in Sleep Architecture
One of the most well-documented findings in sleep research is the progressive decline in deep sleep across the lifespan. A healthy 20-year-old typically spends 15-20% of total sleep time in slow-wave deep sleep. By age 60, that figure drops to 5-10%, and some older adults show virtually no stage N3 activity on polysomnography recordings. This decline is not a sign of disease — it appears to be an intrinsic feature of neural aging, driven by changes in cortical gray matter volume and reduced amplitude of the delta waves that define deep sleep.
REM sleep follows a different trajectory. While there is a modest decline with age, REM percentages remain relatively stable through middle adulthood, typically staying between 20-25% of total sleep time until the seventh decade. The more significant age-related change in REM sleep is fragmentation — older adults tend to have shorter, more interrupted REM episodes rather than fewer of them. This distinction matters because fragmented REM may impair memory consolidation even when total REM minutes appear adequate on a tracker readout.
Understanding these age-related shifts is critical for interpreting sleep tracker data. A 55-year-old who sees 8% deep sleep on their Oura Ring is not necessarily sleeping poorly — they may be sleeping normally for their age. Consumer trackers rarely adjust their benchmarks for age, which can create unnecessary anxiety. The more useful comparison is your own trend over time rather than a population average that may not apply to your demographic.
How Medications and Substances Alter Sleep Stages
Many common medications profoundly alter the balance between REM and deep sleep, often without patients or even prescribing physicians fully appreciating the trade-offs. SSRIs and SNRIs — the most widely prescribed antidepressants — are potent REM suppressors. Patients taking sertraline or venlafaxine commonly show REM percentages of 10-15%, well below the normal range, with a corresponding increase in lighter NREM stages. Whether this REM suppression contributes to the emotional blunting some patients report remains an active area of research.
Benzodiazepines and their close relatives (the "Z-drugs" like zolpidem and eszopiclone) increase total sleep time but suppress both deep sleep and REM sleep in favor of stage N2 light sleep. This is why patients taking these medications often report sleeping longer but not feeling proportionally more rested — the sleep architecture has shifted toward less restorative stages. Beta-blockers, commonly prescribed for hypertension, suppress melatonin production and can reduce REM sleep by 20-30%, potentially explaining the vivid dreams and nighttime awakenings some patients experience when they discontinue these medications.
Cannabis presents a particularly nuanced case. THC appears to increase deep sleep in the short term while suppressing REM sleep — a trade-off that some users find subjectively beneficial. However, chronic use leads to tolerance of the deep sleep enhancement while the REM suppression persists, and cessation triggers a "REM rebound" characterized by intensely vivid, often disturbing dreams. CBD alone does not appear to significantly alter sleep architecture at typical consumer doses, though research remains limited.
How to Improve Deep Sleep and REM Sleep Naturally
Improving deep sleep and REM sleep requires different strategies because the two stages respond to different physiological triggers. Deep sleep (slow-wave sleep) is primarily driven by sleep pressure — the accumulation of adenosine in the brain during waking hours. The most reliable way to increase deep sleep is to extend the waking period before sleep: going to bed slightly later rather than earlier, avoiding daytime naps longer than 20 minutes, and engaging in moderate-to-vigorous exercise at least four to six hours before bedtime. Resistance training appears to be particularly effective — a 2022 meta-analysis in Sleep Medicine Reviews found that regular strength training increased slow-wave sleep by 15 to 20 percent compared to aerobic exercise alone.
REM sleep responds to different levers. Because REM periods grow longer and more frequent in the second half of the night, the single most impactful change for REM enhancement is simply sleeping long enough to access those later cycles — typically requiring seven to eight hours of total sleep. Alcohol is the most common REM suppressant: even two standard drinks consumed within three hours of bedtime reduce REM sleep by 20 to 40 percent in polysomnography studies, a suppression that persists even after the sedative effect wears off. Cannabis similarly suppresses REM sleep during use and causes a rebound of intense, vivid dreaming upon cessation. For anyone prioritizing cognitive consolidation, emotional processing, or creative problem-solving — all REM-dependent functions — eliminating evening alcohol consumption is the highest-impact single intervention available.
How Age Affects the Balance Between REM and Deep Sleep
The proportion of time spent in each sleep stage shifts significantly across the lifespan, and understanding these changes helps set realistic expectations about sleep quality as you age. Young adults typically spend 20 to 25 percent of total sleep time in deep sleep and 20 to 25 percent in REM sleep. By age 60, deep sleep often declines to 5 to 10 percent of total sleep time — a reduction that is biologically normal but frequently misinterpreted as a sleep disorder when viewed through a tracker that compares readings against younger population averages.
REM sleep, by contrast, remains relatively stable throughout adulthood, typically declining by only 2 to 5 percentage points between ages 30 and 70. This asymmetric aging pattern has practical implications: older adults who feel unrested despite sleeping seven to eight hours are likely experiencing the natural reduction in deep sleep rather than a correctable sleep problem. Strategies that specifically promote deep sleep — including regular aerobic exercise, maintaining a cool bedroom, and reducing alcohol consumption — become increasingly important after age 40, when the decline in slow-wave sleep begins to accelerate.
Medications and Substances That Alter Sleep Stage Distribution
Many commonly used substances shift the balance between REM and deep sleep in ways their users do not realize. Alcohol is the most widespread sleep-stage disruptor: it increases deep sleep during the first half of the night while nearly eliminating REM sleep during the second half, producing a net deficit in the memory consolidation and emotional processing that REM sleep provides. Antidepressants — particularly SSRIs and SNRIs — are potent REM suppressors, reducing REM sleep by 30 to 50 percent in most patients. This suppression is one reason vivid dreams and nightmares are common during antidepressant discontinuation, as the brain compensates with a rebound increase in REM activity.
Cannabis suppresses both REM sleep and dream recall, which some users perceive as a benefit (fewer nightmares) but which comes at the cost of reduced overnight memory consolidation and emotional regulation. Melatonin supplements, at doses above 1 milligram, can increase REM sleep duration in some individuals — a potentially useful effect for older adults whose REM percentage has declined, but one that may produce unwanted vivid dreaming at higher doses. If you take any medication or supplement regularly and have noticed changes in dream intensity, sleep quality, or morning alertness, the substance's effect on sleep staging is worth investigating with your prescriber.
Can You Increase One Without Decreasing the Other?
In general, interventions that increase total sleep time tend to increase both deep and REM sleep proportionally, because the brain allocates stages based on its homeostatic needs. However, some interventions selectively favor one stage:
To increase deep sleep: Exercise (especially morning or afternoon aerobic exercise), avoid alcohol and late caffeine, maintain a cool bedroom (deep sleep is temperature-sensitive — a room above 75°F reduces N3), and consider whether you are getting to bed early enough for the front-loaded N3 cycles to occur.
To increase REM: Extend total sleep time (add 30 minutes by going to bed earlier), eliminate alcohol (which suppresses REM in addition to deep sleep), review medications that are known REM suppressants (SSRIs, some antihypertensives, cannabis), and maintain consistent wake times (REM is circadian-timed and benefits from regular scheduling).
Both stages are necessary for full sleep restoration, and neither can compensate for the other's absence. Deep sleep without adequate REM leaves emotional memories unprocessed and motor skills unconsolidated. REM without adequate deep sleep leaves the body under-repaired and the declarative memory system under-maintained. A night that delivers both in age-appropriate proportions — roughly 15 to 20% deep sleep and 20 to 25% REM for adults under 50 — is the target worth tracking toward.
