Guide · Sleep

Deep sleep: how much you need and how to improve it

The stages of sleep, how much deep sleep is normal by age and what increases it, according to studies.

In this article

1. What deep sleep is
2. How much deep sleep is ideal
3. Deep sleep by age
4. Deep sleep vs REM: which matters more
5. How to improve deep sleep
6. Can a ring measure deep sleep?
7. How to track deep sleep without obsessing
8. Frequently asked questions
9. Sources

What deep sleep is

Sleep is not a single state. Through the night, the brain goes through cycles of about 90 to 110 minutes, and each cycle has different stages EV A:

• N1, the transition between wakefulness and sleep, the shortest stage.
• N2, light sleep, which takes up most of the night.
• N3, deep sleep, also called slow-wave sleep, because the electroencephalogram shows large, slow waves.
• REM, the stage of the most vivid dreams, with the eyes moving rapidly under the eyelids.

In deep sleep it is harder to wake up, breathing becomes regular and the body reaches its lowest point of activity in the night. In this stage the brain reactivates and reorganises what was learned during the day, moving part of those memories into long-term storage EV A.

There is also a line of research on brain clearance. In mice, sleep enlarged the space between brain cells and sped up the removal of waste such as beta-amyloid EV C. It is a finding in animals, still under investigation in humans.

How much deep sleep is ideal

Sleep societies recommend 7 to 9 hours a night for adults and 7 to 8 hours for people over 65 EV A. The American Academy of Sleep Medicine and the Sleep Research Society reach a similar conclusion: 7 hours or more, regularly EV A.

For deep sleep itself there is no official target in minutes. Physiology references describe a typical adult night like this EV A:

This means that in an 8-hour night, deep sleep comes to about two hours in a young adult. In someone who sleeps 6 hours, the total falls with it. That is why the first lever for deep sleep is total sleep time.

The number also varies from person to person and night to night. More useful than chasing an exact value is watching your trend: how your deep sleep behaves in weeks with training, stress, travel or afternoon coffee.

Deep sleep by age

Age is the factor that changes deep sleep the most. A meta-analysis of 65 studies with 3,577 people aged 5 to 102 showed that the percentage of slow-wave sleep falls consistently from childhood to adulthood and keeps falling over the years EV A.

In the same study, total sleep time and sleep efficiency also decreased in adults, while lighter stages (N1 and N2) took up more room EV A. An important detail: the effects of age were smaller in studies that excluded people with sleep apnoea, depression, medication use and other conditions. Part of what we call age-related sleep is actually sleep affected by things that can be addressed.

In practice, a 50-year-old should not compare their deep sleep with that of a 25-year-old. The fair comparison is with yourself, over time.

Deep sleep vs REM: which matters more

Both stages are necessary and do complementary work. Deep sleep dominates the first half of the night; as the night goes on, REM periods get longer EV A. Cutting sleep short at the end of the night mainly costs REM. Going to bed very late and waking up early costs a little of everything.

In memory consolidation, research describes a sequence: in slow-wave sleep, recent memories are reactivated and transferred to long-term networks; the REM that follows seems to stabilise them EV A.

Light sleep (N2) also has a role. It takes up almost half the night and bridges the cycles. Having plenty of light sleep is normal and does not mean a bad night.

How to improve deep sleep

What has the most evidence:

• Sleep enough, at regular times. Deep sleep comes first in the night and depends on total time. Going to bed at similar times protects that window.
• Exercise. A meta-analysis of 66 studies found a small, favourable effect of exercise on slow-wave sleep after a session, and effects on total time and sleep quality with regular practice EV A.
• Watch the timing of coffee. 400 mg of caffeine taken up to 6 hours before bed already disrupted sleep in a controlled study EV B. A meta-analysis of 24 studies showed that caffeine reduced total sleep by 45 minutes and shortened deep sleep EV A. Sensitivity varies from person to person.
• Be careful with alcohol. It can increase deep sleep in the first half of the night, but it fragments the second half and reduces REM EV A. The night feels heavier and restores less.

If you snore loudly, wake up choking or feel very sleepy during the day even after sleeping well, talk to a doctor. Disorders such as sleep apnoea change the architecture of sleep and can be treated.

Can a ring measure deep sleep?

It measures an estimate. The standard for identifying sleep stages is polysomnography, a test that records the brain's electrical activity during the night. Wearables use movement, heart rate, temperature and algorithms to infer the stages.

A study comparing seven consumer devices with polysomnography showed high sensitivity for detecting sleep but inconsistent results in splitting it into stages EV B. In one ring, four-stage classification (light, deep, REM and wake) agreed with polysomnography 79% of the time when the algorithm combined movement, autonomic nervous system signals and circadian rhythm EV C.

The right reading is this: a ring is very good for following trends over weeks and less precise for pinning down the minutes of a single night.

How to track deep sleep without obsessing

1. Foundation: make sure you have 7 to 9 hours in bed, at regular times.
2. Baseline: observe 2 to 3 weeks to learn your pattern.
3. One change: change one thing at a time, such as coffee timing, training or alcohol.
4. Trend: compare weekly averages, not single nights.
5. The next day: see how focus and energy respond to sleep.

Last night's sleep shows up the next day, and attention is one of the first signs. A meta-analysis of 70 studies showed that sleep deprivation most strongly affects lapses in simple attention, those moments when the response simply does not come EV A.

Frequently asked questions

Sources

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2. Hirshkowitz M et al. National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health, 2015. doi.org/10.1016/j.sleh.2014.12.010 · Evidence A
3. Watson NF et al. Recommended Amount of Sleep for a Healthy Adult: joint consensus statement of the AASM and SRS. Sleep, 2015. doi.org/10.5665/sleep.4716 · Evidence A
4. StatPearls. Physiology, Sleep Stages. NCBI Bookshelf, atualizado em 2024. www.statpearls.com/point-of-care/29141
5. Rasch B, Born J. About sleep's role in memory. Physiological Reviews, 2013. doi.org/10.1152/physrev.00032.2012 · Evidence A
6. Xie L et al. Sleep drives metabolite clearance from the adult brain. Science, 2013. doi.org/10.1126/science.1241224 · Evidence C
7. Kredlow MA et al. The effects of physical activity on sleep: a meta-analytic review. Journal of Behavioral Medicine, 2015. doi.org/10.1007/s10865-015-9617-6 · Evidence A
8. Drake C et al. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 2013. doi.org/10.5664/jcsm.3170 · Evidence B
9. Gardiner C et al. The effect of caffeine on subsequent sleep: a systematic review and meta-analysis. Sleep Medicine Reviews, 2023. doi.org/10.1016/j.smrv.2023.101764 · Evidence A
10. Ebrahim IO et al. Alcohol and sleep I: effects on normal sleep. Alcoholism: Clinical and Experimental Research, 2013. doi.org/10.1111/acer.12006 · Evidence A
11. Chinoy ED et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep, 2021. doi.org/10.1093/sleep/zsaa291 · Evidence B
12. Altini M, Kinnunen H. The Promise of Sleep: a multi-sensor approach for accurate sleep stage detection using the Oura Ring. Sensors, 2021. doi.org/10.3390/s21134302 · Evidence C
13. Lim J, Dinges DF. A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychological Bulletin, 2010. doi.org/10.1037/a0018883 · Evidence A