How Much Deep Sleep Do You Need?

There is no agreed number of minutes of deep sleep that every adult needs. Sleep guidelines set a target for total sleep, at least seven hours a night for adults, and deep sleep is a share of that, larger in young adults and often smaller in midlife and later. A tracker's deep sleep figure is also its least reliable number.

What is deep sleep?

Deep sleep is stage 3 of non-REM sleep, also called slow wave sleep. The US National Institute of Neurological Disorders and Stroke describes it as the period of deep sleep you need to feel refreshed in the morning. It comes in longer spells in the first half of the night, your heart rate and breathing fall to their lowest levels, and you can be hard to wake (NINDS).

Through the night you cycle through light sleep, deep sleep and REM sleep several times, with longer REM periods later on (NINDS). That is why the early hours feel lighter, which we cover in why you wake at 3am.

What does deep sleep do?

Deep sleep is closely tied to growth hormone. In a study of 149 healthy men, the amount of growth hormone released was linked to the amount of slow wave sleep, independent of age (Van Cauter, 2000).

Sleep also appears to help clear waste from the brain. In mice, natural sleep widened the spaces between brain cells by 60% and sped up the clearance of beta-amyloid, a protein linked to Alzheimer's disease (Xie, 2013). That was an animal study, and how much it applies to people is still being worked out.

How much deep sleep is normal at my age?

It depends on the study. In 149 healthy men, deep sleep made up 18.9% of the night at age 16 to 25 and 3.4% at 36 to 50, with little further fall after that (Van Cauter, 2000). A review of 65 studies also found the share of slow wave sleep falls with age in adults (Ohayon, 2004).

A larger 2019 review of 169 studies and 5,273 healthy adults, scored with current rules, found something different. Total sleep fell by about 10 minutes and time awake in the night rose by about 10 minutes with each decade, but the share of deep sleep did not change significantly (Boulos, 2019).

The practical point is the same either way. A wide range is normal, and the amount that matters most is total sleep. The American Academy of Sleep Medicine and Sleep Research Society advise that "Adults should sleep 7 or more hours per night on a regular basis to promote optimal health" (AASM and SRS, 2015). The US National Sleep Foundation suggests 7 to 9 hours for adults and 7 to 8 for older adults (Hirshkowitz, 2015).

Both short and long sleep are linked with a higher risk of death in long-term studies (Cappuccio, 2010). We look at what that means for long-term health in sleep and long-term health outcomes.

Can I trust my tracker's deep sleep figure?

Treat it with caution. In a 2024 lab study, the Apple Watch Series 8 underestimated deep sleep by 43 minutes a night and the Fitbit Sense 2 by 15 minutes, while the Oura Ring Gen3 did not differ significantly from polysomnography (Robbins, 2024). A low reading may say more about the device than about you. Our guide to sleep tracker accuracy covers each device.

What helps deep sleep?

Exercise is one of the better-studied options. A review of 66 studies found a single session of exercise had a small benefit on slow wave sleep, and regular exercise had moderate benefits on overall sleep quality (Kredlow, 2015).

Alcohol is a trap. It tends to increase deep sleep early in the night and disturb sleep in the second half (Ebrahim, 2013). A higher deep sleep score after a few drinks is not a better night.

A consistent wake time helps sleep settle into a pattern (AASM, 2021). And if your sleep is broken by snoring or pauses in breathing, sleep apnoea may be fragmenting it, one of the features NICE lists as a reason to assess for it (NICE NG202).

When is it worth seeing a doctor?

See a doctor if you sleep seven hours or more but still wake unrefreshed, feel sleepy in the day, snore loudly, or have been told you stop breathing in your sleep. Those are reasons to look for a cause, whatever your tracker says.

Getting answers at The Wellness

At our Sleep Clinic, a Sleep Consultation costs £450 and ends with a clear plan, including whether a test would help. Where it would, the Home Sleep Study is £1,395. For people who want the whole picture, The Sleep Clinic programme, the one most people choose, is £2,995 and adds a consultant, blood tests chosen for your sleep and a three-month review.

Short sleep has a cost of its own. People who sleep under six hours a night have a 10% higher risk of death than those sleeping seven to nine, according to RAND Europe (RAND Europe, 2016). Book a sleep consultation if your nights are not doing their job.

This article is general information and does not replace a consultation. If you are falling asleep at the wheel, stop driving until you have been assessed. If you have chest pain, severe breathlessness or signs of a stroke, call 999.