Deep sleep: how much do you need and how do you increase it?

Deep sleep takes up roughly 13 to 23 percent of the night in healthy adults, and you get most of it in the first half. It is the stage where growth hormone release peaks, so it is the stage your recovery leans on most. You cannot force it, but you can protect it.

The numbers most Dutch sleep blogs quote come from mattress shops. The numbers below come from research.

How much deep sleep do you need?

For adults, deep sleep usually adds up to roughly one and a half to two hours a night, depending on your total sleep time. More important than a target number is that you cannot dial it up yourself. Your body takes the deep sleep it needs, as long as you give it the time.

Sleep six hours instead of eight and you mostly cut your later cycles. Those are relatively rich in dream sleep, so your deep sleep initially holds up reasonably well. That is exactly why your tracker can reassure you about your deep-sleep percentage while you are still sleeping too little.

A percentage says little when neither the numerator nor the denominator is right.

Why does deep sleep decline with age?

Because the amount of deep sleep falls sharply through your twenties and thirties. In a JAMA study of 149 healthy men, the share of deep sleep dropped from 18.9 percent in early adulthood (16 to 25 years) to 3.4 percent in midlife (36 to 50 years). Growth hormone release declined at the same pace (PMID 10938176).

Read that number again. Not slightly less: from nearly a fifth of your night to a little over three percent.

The same study showed dream sleep only drops off later in life, and that evening cortisol rises after age fifty by about 19.3 nmol/l per decade. So your sleep changes in two waves, with different hormones behind each.

Picture two lifters of 24 and 44 who both spend eight hours in bed: on paper they sleep the same, but in deep sleep the gap is roughly fivefold. The older one waking up less rested is not imagination, and it is not failure.

What happens in your body during deep sleep?

Your brain waves slow, your heart rate and breathing drop, and your pituitary releases growth hormone in pulses. For an athlete, that is the stage where muscle repair and bone building get their best conditions. Get woken out of it and you feel groggy: that is the familiar sleep inertia.

The link between deep sleep and growth hormone was strong in the JAMA study, independent of age (PMID 10938176). Less deep sleep went together with less growth hormone, no matter how old someone was.

That is why I take sleep more seriously than most of the supplements in my cupboard. No powder puts your pituitary to work as reliably as a dark room and seven to nine hours of time.

How do you increase your deep sleep?

Not with a trick, but with the conditions. More total sleep time, a fixed rhythm, a cool room and less alcohol simply give your body more opportunity to take deep sleep. There is no button that lengthens the stage itself.

Alcohol is the biggest disruptor here. It gets you to sleep faster, but your night turns choppier and shallower in its second half.

The Hersenstichting, the Dutch brain foundation, stresses that sleep quality matters at least as much as sleep duration. To me that leads to a practical conclusion: steer your bedtime, not your stage percentages.

What magnesium does and does not do for your night is covered in our piece on magnesium for athletes.

Can a tracker measure deep sleep reliably?

Not precisely. Consumer trackers are good at the question of whether you are asleep, but considerably weaker at which stage you are in. In a comparison of seven devices against polysomnography, staging was inconsistent and got worse during disrupted sleep (PMID 33378539).

So use the number as a trend across weeks, not as a report card per night. If your watch says your deep sleep fell from 1h20 to 1h05, that sits inside the device error margin.

The full explanation is in sleep stages explained and in sleep tracker versus blood values.

What does more sleep get you in the gym?

In basketball players who extended their sleep by an average of 111 minutes a night for six weeks, sprint time improved from 16.2 to 15.5 seconds and free throws rose by 9 percent (PMID 21731144). That is a bigger jump than most athletes get out of a training block.

They did nothing else. They stayed in bed longer.

If you want to see where your recovery stands, our 360 health blood test measures cortisol, glucose and testosterone among others. The rest of the approach is in our pillar on better sleep for athletes.

My advice: add 30 minutes to your time in bed this month and leave your stage percentages alone.

References

1. 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. PMID 10938176.
2. Mah CD, Mah KE, Kezirian EJ, Dement WC. The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep. 2011. PMID 21731144.
3. Chinoy ED, Cuellar JA, Huwa KE, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep. 2021. PMID 33378539.
4. Hersenstichting. Sleep cycle and sleep tips. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.