Deep Sleep and REM Sleep: How Much Do You Need?

The stages of sleep

Sleep moves through repeating cycles. Each cycle contains non-REM sleep, which has three stages, followed by REM sleep.¹

• Stage 1. The brief drift from wakefulness into sleep
• Stage 2. Light sleep. Heart rate and breathing slow and body temperature drops. You spend more of the night here than in any other stage
• Stage 3. Deep sleep, also called slow-wave sleep. It is the hardest stage to be woken from and the one you need to feel refreshed in the morning
• REM sleep. Rapid eye movement sleep, when most vivid dreaming happens. The brain is active and the limb muscles are temporarily switched off

REM sleep first appears about 90 minutes after you fall asleep. Deep sleep is concentrated in the first half of the night, and REM periods get longer towards morning.¹

What is core sleep?

“Core sleep” is a label used by some sleep trackers, including Apple Watch, for the lighter stages of non-REM sleep, stages 1 and 2. It isn’t a medical term, and it doesn’t mean essential sleep. It is normal for it to make up the largest share of your night.

How much deep sleep do you need?

There is no recommended amount. Guidelines set a target for total sleep, not for individual stages.⁷

What researchers can say is what is typical. In healthy adults, deep sleep makes up roughly 10 to 20 per cent of the night, which is about one to one and a half hours of a seven to eight hour sleep. A meta-analysis covering sleep from childhood to old age found that with age, the share of lighter sleep rises and deep and REM sleep shrink.²

The drop in deep sleep is steep and starts early. In a study of healthy men, deep sleep fell from about 19 per cent of the night in early adulthood to about 3 per cent by midlife.³ Less deep sleep at 50 than at 20 is expected, not a disorder.

How much REM sleep do you need?

Again, there is no set target. REM typically makes up about 20 to 25 per cent of an adult’s night, or roughly one and a half to two hours, and it declines more gradually with age than deep sleep does.^2,3

What deep sleep and REM sleep do

• Deep sleep is when the body releases most of its growth hormone, which supports tissue repair.³ Animal research suggests the brain also clears waste products more efficiently during sleep.⁴ Deep sleep plays a central part in fixing new facts and events into long-term memory⁵
• REM sleep contributes to memory in a different way, including skills and the emotional side of memories⁵

Can you trust your sleep tracker?

For total sleep time, mostly. For stages, not really. When seven consumer devices were tested against a full sleep study, they were good at telling sleep from wake, but their estimates of light, deep and REM sleep were inconsistent.⁶

A low deep sleep score on a watch is not a diagnosis. If you feel rested, the number matters very little.

How to get more deep sleep and REM sleep

You can’t target a stage directly. What works is protecting sleep as a whole.

• Get enough total sleep. Seven or more hours for adults. Cutting sleep short at either end removes REM in particular, because most of it comes late in the night^1,7
• Keep regular hours. A steady wake time keeps your sleep cycles lined up with your body clock
• Exercise regularly. Physical activity improves sleep quality and depth⁸
• Watch caffeine and alcohol. Caffeine six hours before bed still reduces sleep, and alcohol breaks up sleep in the second half of the night^9,10

When to see a doctor

• You sleep enough hours but wake unrefreshed most days
• Loud snoring or pauses in breathing. Sleep apnoea fragments deep and REM sleep
• You act out dreams, shouting or lashing out in your sleep
• You are sleepy enough during the day to doze off unintentionally

References

1. National Institute of Neurological Disorders and Stroke (US). Brain basics: understanding sleep.
2. Ohayon MM, Carskadon MA, Guilleminault C, et al. Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals: developing normative sleep values across the human lifespan. Sleep. 2004;27(7):1255-73.
3. Van Cauter E, Leproult R, Plat L. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA. 2000;284(7):861-8.
4. Xie L, Kang H, Xu Q, et al. Sleep drives metabolite clearance from the adult brain. Science. 2013;342(6156):373-7.
5. Rasch B, Born J. About sleep's role in memory. Physiol Rev. 2013;93(2):681-766.
6. Chinoy ED, Cuellar JA, Huwa KE, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep. 2021;44(5).
7. Watson NF, Badr MS, Belenky G, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015;38(6):843-4.
8. Kredlow MA, Capozzoli MC, Hearon BA, et al. The effects of physical activity on sleep: a meta-analytic review. J Behav Med. 2015;38(3):427-49.
9. Drake C, Roehrs T, Shambroom J, et al. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195-200.
10. Sleep Health Foundation. Caffeine, food, alcohol, smoking and sleep.