Neither REM nor deep sleep is better than the other. They handle fundamentally different jobs, and your body needs both every night. Deep sleep restores your body physically, while REM sleep processes emotions and strengthens memory. Losing either one creates distinct health problems that the other stage can’t compensate for.
Understanding what each stage actually does can help you figure out which one you might be short on and what to do about it.
What Deep Sleep Does for Your Body
Deep sleep, also called slow-wave sleep, is the most physically restorative stage. During this phase, your body repairs tissues, builds bone and muscle, and strengthens the immune system. Growth hormone and appetite-regulating hormones are released primarily during deep sleep, which is why poor sleep is so tightly linked to weight gain.
Your brain also performs critical maintenance during deep sleep. Slow oscillating brain waves create pulses of cerebrospinal fluid through the brain’s waste-clearance system (called the glymphatic system), flushing out harmful metabolic byproducts. This clearance happens specifically during deep sleep, not during REM or lighter stages. The buildup of these waste proteins is associated with neurodegenerative diseases, which is one reason researchers pay close attention to deep sleep quality in aging adults.
When deep sleep is suppressed, the metabolic consequences show up fast. In one study of healthy men, a single night of deep sleep suppression reduced insulin sensitivity by 15%. Research on over 2,700 older men found that those with the lowest percentage of deep sleep had the highest BMI and largest waist circumference. These findings suggest deep sleep plays a direct role in how your body handles blood sugar and stores fat.
What REM Sleep Does for Your Mind
REM sleep is when your brain consolidates emotional memories and, in a sense, performs “overnight therapy.” During REM, your brain replays emotionally charged experiences from the day but strips away some of the raw emotional intensity while preserving the factual content. This is why a bad day often feels more manageable after a good night’s sleep. The specific brainwave patterns during REM help integrate new emotional experiences into your existing long-term memory networks.
The relationship between REM sleep and mood is complex. People who consistently get too little REM sleep tend to have more difficulty regulating emotions and may be more reactive to stress. Interestingly, selectively depriving depressed patients of REM sleep has been shown to provide dramatic, immediate (though temporary) relief from depressive symptoms in some cases. This doesn’t mean REM sleep causes depression. It suggests that REM sleep actively processes emotional material, and in people with mood disorders, that processing can sometimes reinforce negative emotional patterns.
How They’re Distributed Through the Night
Deep sleep and REM sleep don’t compete for the same time slots. They naturally concentrate in different parts of the night. You get the majority of your deep sleep in the first half, particularly in the first two or three sleep cycles. REM periods start short and grow longer as the night progresses, so most of your REM sleep happens in the last few hours before waking.
This distribution matters practically. If you go to bed late but still wake up at a normal time, you’re mostly cutting into REM sleep. If you fall asleep on time but wake up too early, you lose REM-heavy cycles at the end. If alcohol or other disruptions fragment your early sleep, deep sleep takes the bigger hit.
In a healthy adult, deep sleep makes up about 25% of total sleep time, REM sleep accounts for another 25%, and the remaining 50% is lighter sleep stages. Stage 2 light sleep alone takes up roughly 45% of the night, so don’t be alarmed if your sleep tracker shows a large chunk of “light sleep.” That’s normal architecture.
Why Deep Sleep Gets Harder With Age
Adults older than 60 can lose up to 70% of their deep sleep compared to adults 25 and younger. This is one of the most significant age-related changes in sleep and may partly explain why older adults face higher rates of metabolic disease, cognitive decline, and immune vulnerability. REM sleep also decreases with age, but the drop in deep sleep is more dramatic and begins earlier.
If you’re in your 40s or older and your sleep tracker shows very little deep sleep, that’s worth paying attention to, but it’s also partly a normal shift. The strategies below can help preserve what deep sleep you still get.
What Disrupts Each Stage
Alcohol is one of the most common REM sleep disruptors. Even moderate drinking suppresses REM sleep, particularly in the first half of the night, by boosting a brain chemical that actively blocks the REM state. One study found that REM sleep in the first half of the night dropped from about 17% at baseline to 7% on drinking nights. The effect persists even with regular use: after nine consecutive nights of drinking, REM sleep remained suppressed.
Deep sleep is more vulnerable to environmental disruption. Being too warm, experiencing noise, or having fragmented sleep from conditions like sleep apnea all reduce deep sleep disproportionately. Chronic stress and elevated cortisol also suppress deep sleep, which creates a vicious cycle since deep sleep helps regulate stress hormones.
How to Support Both Stages
Since the two stages serve different purposes and occupy different parts of the night, the most effective approach is protecting total sleep duration and quality rather than trying to boost one stage specifically.
- Keep your bedroom cool. A temperature between 60 and 67°F (15 to 19°C) supports the natural body temperature drop that facilitates deep sleep. Being too warm is one of the most common and easily fixable causes of fragmented sleep.
- Exercise regularly. Aim for about 150 minutes of moderate activity per week plus two strength-training sessions. Regular exercise consistently increases deep sleep duration in research, though exercising too close to bedtime can have the opposite effect for some people.
- Limit alcohol, especially in the evening. Even a couple of drinks within a few hours of bedtime can significantly reduce REM sleep. If you notice vivid or intense dreams on nights you don’t drink, that’s likely REM rebound, your brain catching up on missed REM sleep.
- Protect both ends of your sleep window. Going to bed on time preserves early-night deep sleep. Sleeping long enough to get through later cycles preserves REM. Cutting sleep short on either end disproportionately affects one stage.
If you consistently wake up feeling physically sore or run down despite sleeping enough hours, you may not be getting adequate deep sleep. If you’re emotionally reactive, struggling with memory, or having difficulty concentrating, insufficient REM sleep could be a factor. In either case, the fix usually starts with the same basics: consistent sleep timing, a cool and dark room, and avoiding the substances that interfere with your brain’s ability to cycle through all the stages it needs.