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8th February, 20214 min read

What is deep sleep and is it important?

Medical reviewer: Dr Ann Nainan
Author: Jennifer Co
Last reviewed: 02/02/2021
Medically reviewed

All of Healthily's articles undergo medical safety checks to verify that the information is medically safe. View more details in our safety page, or read our editorial policy.

When it comes to how much sleep you need, everyone is slightly different. It’s generally thought that adults should sleep between 7 and 9 hours a night – though some people may only need 6 hours, while others may need as many as 10.

And sleep quality is just as important as sleep quantity.

When you sleep, your body goes through different stages, and you go through each stage several times during 1 night. Deep sleep is an important stage – so read on to find out why it matters.

What is deep sleep?

Sleep can be divided into 2 main stages: rapid eye movement (REM) and non-rapid eye movement (NREM). NREM sleep also has 3 stages: N1, N2, and N3. You spend most of your sleep time in NREM sleep, and N3 is the deepest level of sleep.

In the minutes after you go to bed and doze off, you’re entering the N1 stage of NREM sleep. Shortly after this, you enter the N2 stage, which is a light sleep where you could still easily be woken up. You’ll usually spend about half of your sleep time in this stage of sleep.

Deep sleep occurs when you reach the N3 stage of NREM sleep. It’s more difficult to be woken up during this stage, and if you do wake up, you may feel confused or ‘groggy’ at first. In this stage of deep sleep, your blood pressure, heart rate and breathing rate are at their lowest and your muscles are relaxed.

After the 3 stages of NREM sleep, you’ll usually have a short stage of REM sleep. This is when you tend to dream – your brain is unusually active, more like when you’re awake – and your blood pressure, heart rate and breathing rate increase.

During a night’s sleep, you’ll go through these stages several times, with a full ‘cycle’ lasting about 90 minutes. This happens 4 or 5 times a night, depending on how long you sleep for.

Why is deep sleep important?

Every stage of sleep is important, but it’s generally thought that deep sleep is the most important because it’s when you get high-quality rest.

Studies suggest that the main function of deep sleep is to give your brain time to recover and restore itself after being active all day. Deep sleep is important for your memory – people with sleep-deprivation often do less well in memory tests – and learning processes.

Growth hormones are also released most during this sleep stage, which are important for the growth and development of your body.

Finally, it’s thought that deep sleep plays an important role in helping your immune system – the defence system that stops you from getting ill – to work properly.

How much deep sleep do you need?

If you're getting 8 hours of sleep a night, you'll typically spend 1 to 2 hours of this in deep sleep.

If you don’t get enough sleep in a night (typically less than 6 hours) over a long period of time, it can start to have a big impact on your health. Long-term sleep loss is associated with a higher risk of:

  • mood disorders
  • obesity
  • type 2 diabetes
  • high blood pressure
  • heart disease
  • heart attack

Interestingly, it seems that both not getting enough sleep and having too much sleep are associated with similar health problems.

How can you get more deep sleep?

The most important way to get more deep sleep is to make sure you’re getting enough total sleep.

The following can help you to sleep well:

  • regular exercise (but not too close to bedtime)
  • body heating, such as a hot bath or sauna, before bed
  • reducing stress
  • having a regular sleep routine
  • minimising stimulus (such as by blocking out noise)

When to see a doctor about sleep

If you often have trouble sleeping, or you wake up feeling tired despite getting the recommended amount of sleep, you may have an underlying condition – so it’s a good idea to speak to your doctor. They will be able to help you work out the cause, and suggest treatment if necessary.

Find out more about when to see a doctor about sleep here.

Key points

  • the quality of sleep you get is as important as the quantity
  • there are 4 main stages of sleep and you go through these stages several times during one night
  • deep sleep is thought to be the most important stage, as it’s when you get high-quality rest
  • you should usually get 1 to 2 hours of deep sleep every night
  • not getting enough sleep over a long period of time can increase your risk of several health conditions


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Deep sleep: How to get more of it [Internet]. 2017 [cited 22 December 2020]. Available here

Sleep and tiredness [Internet]. 2018 [cited 22 December 2020]. Available here

Besedovsky L, Lange T, Haack M. The sleep-immune crosstalk in health and disease. Physiol Rev. 2019;99(3):1325–80.

Brain basics: Understanding sleep [Internet]. [cited 8 January 2021]. Available here

The importance of sleep and understanding sleep stages [Internet]. 2017 [cited 8 January 2021]. Available here

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Oversleeping: bad for your health? [Internet]. John Hopkins Medicine. N.d [cited 12 January 2021]. Availablehere

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Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.

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