How much deep sleep do you need? What UK sources say, and why your tracker may differ

How much deep sleep do you need?

There is no single minimum that the British sources set for you personally. What The Sleep Charity gives is a typical share: deep sleep, which it labels NREM3, makes up 20 to 25% of sleep.¹ That is a description of how sleep is usually divided, not a target you pass or fail.

A worked example

Take a night with 7 hours of actual sleep. That is 7 × 60 = 420 minutes. Twenty per cent of 420 is 84 minutes, or 1 hour 24 minutes. Twenty-five per cent is 105 minutes, or 1 hour 45 minutes. So the typical range for that night is roughly 84 to 105 minutes.¹

Now suppose a tracker shows 58 minutes of deep sleep for the same night. 58 ÷ 420 is about 13.8%, which is below the 20 to 25% range. That is arithmetic, not a verdict. The studies below show that sensor-based stage estimates are inconsistent, so a figure like this is a reason to look at your trend and how you feel, not to worry. The 58 minutes is an invented number.

What counts as deep sleep?

Deep sleep is the third stage of non-REM sleep. The Sleep Charity calls it slow wave sleep and describes it as the most restorative part of sleep, helping the body to repair and recover.¹ The Royal College of Psychiatrists says stage 3 is important for memory processing, mental function and the physical repair of the body.²

During non-REM sleep, the College adds, the body secretes growth hormones into the bloodstream and the brain clears out waste products.² It is also the stage in which sleep terrors and sleepwalking usually happen.²

Deep sleep is not spread evenly. The Sleep Charity says we usually get our deepest sleep earlier in the night and lighter sleep towards the morning.¹ The College agrees: the first couple of cycles hold a lot of deep sleep, and later ones very little.² A tracker chart that shows deep sleep bunched early in the night therefore fits what the sources describe.

For the other stages, and how a 90-minute cycle fits together, see the guide to sleep stages.

Does deep sleep change with age?

It does. The sources give the direction clearly and the exact percentages less so, and this guide gives only what was read.

The Sleep Charity puts it this way: sleep needs stay much the same with age, but sleep quality often declines. Spending less time in stage 3 is part of why sleep can feel less refreshing.³

Why does your watch show less deep sleep than a guide says?

A wrist device estimates stages. It does not record them the way a laboratory does. In a laboratory comparison of seven consumer sleep trackers against polysomnography, sleep stage comparisons were mixed, and the devices tended to perform worse on nights with poorer or disrupted sleep.⁹

A systematic review of Fitbit models reached a related conclusion: the devices give gross estimates of time in sleep stages, are of limited specificity and are not a substitute for polysomnography.¹⁰ Both studies tested particular models, not every device on sale.

That has two practical consequences. A single low reading tells you little. And the nights when you most want a precise stage count, such as broken nights, are the ones where devices tended to do worst.⁹

What can shorten or fragment deep sleep?

The sources read name a handful of things that affect sleep. Only alcohol is tied to deep sleep directly; the rest are listed as causes of poor sleep in general.

• Alcohol. The Sleep Charity says it can stop you getting the deep, restful sleep your body and brain need, and that it also reduces dream sleep.⁵
• A room that is too hot or cold, noise, and alcohol, caffeine or nicotine. The NHS lists these among the most common causes of insomnia.⁶
• Temperature. The Sleep Charity gives an ideal bedroom temperature of around 16 to 18 °C. Above 24 °C is likely to cause restlessness, and about 12 °C makes it hard to drop off.⁸

Alcohol and caffeine timing: the sources differ

The NHS gives two different timings, so this guide shows both rather than picking one. Its insomnia page says not to smoke or drink alcohol, tea or coffee for at least 6 hours before bed.⁶ A separate NHS Every Mind Matters page says alcohol and nicotine can disrupt sleep and suggests avoiding them 1 to 2 hours before bed.⁷ For a bedtime of 23:00, 6 hours before is 17:00, while 1 to 2 hours before is 21:00 to 22:00.

The two pages also list different substances: the insomnia page includes smoking, tea and coffee, while the Every Mind Matters advice names alcohol and nicotine. This guide shows both and does not choose between them.

What can you change? Habits the NHS lists

No source read promises that any habit raises the amount of deep sleep, and this guide does not either. What the NHS gives are habits for sleeping better in general.

• Get out of bed at the same time every day, and go to bed only when you feel sleepy.⁶
• Keep the bedroom quiet, dark and cool, and relax before bed.⁷⁶
• Cut back on alcohol, caffeine and nicotine, using the timings above.⁶⁷

For the full list, with times worked out for a sample bedtime, see how to improve your sleep quality.

A 14-night way of checking your own trend

This is a simple method of our own, not one from a health source. Keep the same bedtime for 14 nights, change one habit only, and note the deep sleep figure from the same tracker each morning. Use the middle value (the median) of the first seven nights and of the last seven, not the average, so one odd night does not pull the result. A shift smaller than the usual night-to-night spread is not evidence of a change.

Deep sleep versus REM sleep

Deep sleep and REM sleep are different jobs, not rivals. Deep sleep is linked by the Royal College of Psychiatrists to memory processing, mental function and physical repair,² while The Sleep Charity links REM sleep to learning, memory and emotional health.¹ Neither source ranks them. You can read more in the sleep stages guide.

Frequently asked questions