What is Deep Sleep & Why is it Important?

You must have heard at some point or the other that adults need at least eight hours of sleep every night. However, while most people lay emphasis on the quantity of sleep, the quality of sleep you get is also equally important. As you sleep, the body goes through various stages of the sleep cycle. One of the critical stages of your sleep cycle is known as deep sleep. Also known as slow-wave sleep, deep sleep is the important stage in your sleep cycle that you need to wake up in the morning feeling refreshed. Unlike the REM (rapid eye movement) sleep stage, deep sleep is the stage when your brain waves and body slow down to enable proper brain function and memory. Read on to find out about what is deep sleep and why it is important.

Deep sleep, or slow-wave sleep, is an important stage of your sleep cycle that you need to wake up feeling refreshed in the morning. Deep sleep is sometimes also referred to as delta sleep. Unlike the more commonly known rapid eye movement (REM) sleep, the deep sleep stage is when your brain waves and body slows down. This important stage of the sleep cycle allows for proper brain function and memory. So while most adults typically focus on getting seven to nine hours of sleep every night, the quality of sleep is also quite important.^{(1, 2, 3, 4, 5)}

There are two main categories of sleep, known as rapid eye movement (REM) sleep and non-REM sleep, and each of them further has important stages. An adult cycles through these stages in more or less a regular pattern as they sleep. Getting a full night’s rest means that the body has gone through these stages a few times before waking up in the morning. There are several ways to not only get better sleep but also more deep sleep every night, which allows you to wake up in the morning feeling more refreshed and well-rested. When you are in a deep sleep, it can be challenging to wake up from that stage, and if you do wake up from deep sleep, you are likely to feel incredibly groggy.^{(6, 7)}

As mentioned above, sleep is divided into two major categories – REM and non-REM sleep. Once you go to sleep, the sleep cycle begins with non-REM sleep, followed by a period of REM sleep. This cycle then continues throughout the night for about every 90 minutes.⁽⁸⁾ Deep sleep takes place in the final stage of the non-REM sleep cycle.

Let us take a closer look at these stages:

The first stage of non-REM sleep lasts for a few minutes as you transition from being awake to going to sleep. During stage one of non-REM sleep, the following happens:

• Your bodily functions like respiration, heartbeat, and eye movements start to slow down.
• Your brain waves start to slow down from being in their ‘awake’ state.
• Your muscles also start to relax, and there are only a few occasional twitches.

Stage two of non-REM sleep lasts for nearly 50% of the total sleep cycle, and this is the stage of sleep that you are likely to fall into more than the other stages during the night.⁽⁹⁾ During stage two, the following happens:

• Your core temperature goes down.
• Your eye movements stop.
• The bodily systems continue to slow down and relax.
• The brain waves have slowed down, but there are still some short bursts of activity remaining in the brain.

Stages three and four of non-REM sleep are when you actually go into a deep sleep. The following happens during these stages:

• The brain waves become the slowest they ever will be during your sleep.
• Your breathing and heartbeat are also at their slowest as your muscles relax.
• It is difficult to wake up a person at this stage, even with loud noises.

The first stage of deep sleep can last for anywhere between 45 to 90 minutes. It has been found that this stage tends to last for longer periods of time during the first half of the night and starts to get shorter with each sleep cycle. ^{(10, 11)}

When you reach stage five that is when the first stage of REM sleep occurs. This happens around 90 minutes after you move through the non-REM sleep stages.^{(12, 13)} During this stage, you experience the following:

• The eyes move rapidly from side to side.
• Your breathing becomes faster and may even be irregular at times.
• You dream as your brain activity increases towards a more wakeful state.
• Your heart rate increases to near a wakeful state.
• You may even feel like your limbs are paralyzed.

There are many benefits of deep sleep. When you are in the stage of deep sleep, glucose metabolism in the brain increases, which supports both short-term and long-term memory and the overall process of learning. ⁽¹⁴⁾

The deep sleep stage is also when the pituitary gland goes to work secreting important hormones like the human growth hormone, which leads to growth and development of the body. This is all the more reason why deep sleep is so important for growing children. ⁽¹⁵⁾

Some of the other benefits of deep sleep may include:

• Cell regeneration
• Restoration of the energy reserves
• Increasing blood supply to the muscles
• Boosting the immune system
• Promoting growth and repair of your tissues and bones

Deep sleep is an important process of the body. Deep sleep is necessary for the body as it helps process the information we come across every day. Without adequate deep sleep, the brain is not able to convert this information to your memory. Not getting enough deep sleep is linked with several medical conditions, including: ^{(16, 17)}

• Night terrors
• Sleep eating
• Bedwetting
• Sleepwalking

At the same time, not getting enough good quality sleep is also known to be linked to conditions like the following:

• Diabetes
• Heart disease
• Alzheimer’s disease
• Stroke

On an average night of sleep, you spend around 75 percent of the time in non-REM sleep, while the other 25 percent of the time is spent on REM sleep. Out of this, just 13 to 23 percent of your total sleep time is considered to be a deep sleep.⁽¹⁸⁾

The stage of deep sleep tends to decrease with age. For example, if you are under the age of 30, you are likely to get two hours of deep sleep every night. On the other hand, a person over the age of 65 may only get around half an hour of deep sleep every night, or it may even be none at all.⁽¹⁹⁾

While there is no specified amount of deep sleep that you need, but typically younger people need more deep sleep because it promotes growth and development of the body. This does not mean, though, that older people do not need deep sleep. As you get older, you still need deep sleep, but not getting as much does not necessarily indicate that you have any sleep disorder. ⁽²⁰⁾

If you find yourself waking up feeling exhausted, it is a good indicator that you are not getting enough deep sleep. Nowadays, there are many types of wearable devices you can wear at home to measure your sleep. These devices track your body’s movements during the night. While the technology is still relatively new, using such devices may help you identify sleeping patterns. However, they are unlikely to be a reliable indicator of just how much deep sleep you are getting. ⁽²¹⁾

If you are really worried about how much deep sleep you are getting or you are experiencing other signs of sleeping trouble, your doctor may recommend that you undertake a sleep study known as a polysomnography (PSG). During a polysomnography test, you will be asleep at a laboratory while being hooked up to monitors. ⁽²²⁾ These monitors will measure the following:

• Heart rate
• Breathing rate
• Body movements
• Oxygen levels
• Brain waves

Your doctor will then use all this information to determine if you are reaching deep sleep and the other stages of sleep during the night.

There are a few ways in which you can increase the amount of deep sleep that you get every night. According to the American Sleep Association, the most important ⁽²³⁾ important thing to do if you want to increase the amount of deep sleep you get every night is to set aside some more time for sleep. By doing this, you allow your body the time to go through more sleep cycles, which in turn makes it possible to get more deep sleep.

Other tips that may help increase deep sleep and good sleep in general include:

• Changing your diet to include lesser carbohydrates and more healthy fats.
• Indulging in more vigorous exercise like jogging, running, or swimming earlier in the day instead of before bedtime.
• Warming up the body in a hot sauna or spa.

Also, there are some antidepressants that may help you achieve deeper sleep, though this is not the case for everyone, and you should never take antidepressants without consulting your doctor first. ²⁴⁾

Pink noise has also been shown to be effective in increasing deep sleep. Pink noise is known as any random noise that has a more low-frequency component as compared to white noise. A study published in the Frontiers in Human neurosciences journal examined the effects of using sound stimulation with pink noise on deep sleep. The results showed that listening to pink noise can enhance a person’s deep sleep state, leading to better memory function once they wake up.⁽²⁵⁾

Getting a good quantity and quality of sleep are both equally important. Deep sleep is an important part of your overall sleep. There are some ways to promote deeper sleep, including listening to pink noise while going to sleep or tiring out the body through exercise. One of the best and most natural ways to get more deep sleep is just to set aside more time to sleep every night. This allows your body some extra time to go through more sleep cycles, thus increasing the chances of having more deep sleep stages.

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