Slow wave or deep sleep is a stage in the sleep cycle necessary for proper brain function and memory. Most people need 7–9 hours of sleep per night, but working out how much deep sleep they need is more complex.

The two main categories of sleep are called rapid eye movement (REM) sleep and non-REM sleep, and each has important stages. People in good health will cycle through these stages in a somewhat regular pattern as they sleep, and a full night’s rest means cycling through these stages a few times before waking up.

There may be some ways to get better quality sleep and more deep sleep each night, allowing a person to wake up feeling more rested and refreshed.

This article explains what deep sleep is and why it is important for health. It also discusses the stages of sleep and how to sleep more deeply.

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Although all the stages of sleep are necessary, deep sleep is especially important for brain health and function.

This stage of sleep helps the brain rest and recover, allowing it to replenish energy. It also plays a role in the reinforcement of declarative memory, or remembering facts.

Deep sleep also contributes to keeping hormones balanced. The pituitary gland secretes human growth hormone during this stage, which helps tissues in the body grow and regenerate cells.

According to the American Academy of Sleep Medicine (AASM), good quality sleep can help by:

  • reducing the frequency of sickness
  • promoting the maintenance of a moderate weight
  • improving mood
  • lowering the risk of health problems, such as heart disease and type 2 diabetes
  • improving decision-making
  • improving brain health and preventing conditions such as dementia

Learn about more benefits of sleep.

The body cycles through the three non-REM stages of sleep:

Stage one

The first stage of the sleep cycle is a transition period during which the body and brain shift from a state of wakefulness to one of sleep. This period is relatively short, lasting only a few minutes, and the sleep is fairly light. People may wake up from this stage of sleep more easily than from other stages.

During stage one, the brain unwinds, along with the body. The monitoring of sleep reveals slow rolling eye movements. The brain waves start slowing down as brain activity and responses to sensory stimulation decrease.

The body also starts to slow its rhythms down during this stage. The heart rate and breathing rate slow, and the muscles start to relax, although they may occasionally twitch.

Stage two

The second stage of non-REM sleep is another lighter stage of sleep that occurs as the body starts transitioning to deeper sleep. Humans spend most of their time during the sleep cycle in this stage of sleep.

In the body, the heart rate and breathing rate slow down even more. The muscles relax further, and eye movements stop. The body temperature also goes down.

Although the brain waves slow down further, this stage also includes small bursts of electrical signals in the brain.

Stage three

Deep sleep, or slow wave sleep, is the third stage of non-REM sleep. Although the body completes a few cycles throughout the night, the third stage occurs in longer periods during the first part of the night.

In the body, the heart rate and breathing rate are at their lowest and most regular during this part of the sleep cycle. The brain waves become even slower and larger.

It may be very difficult to wake someone from this stage of sleep, which is when sleep disorders, such as sleepwalking, occur.

REM sleep is the fourth stage of the sleep cycle. The body first goes into REM sleep about 90 minutes after falling asleep.

During this stage of sleep, the eyes dart back and forth behind the closed eyelids. This state is closer to the wakeful state than the other stages of sleep.

In REM sleep, the brain waves start to resemble the brain waves of the wakeful state. The heart rate and breathing rate speed up, and the rhythms may become irregular.

The REM stage is also when most vivid dreaming occurs. The brain temporarily paralyzes the muscles to prevent the body from acting out these dreams.

Learn more about sleep cycle stages.

Importantly, a person has to get enough deep sleep to allow the brain and body to repair themselves. This will help the person feel restored. The amount of deep sleep an individual has will relate to how much overall sleep they get. Sleeping 7 or more hours is the recommendation for most adults, which will usually give the body plenty of time in the deeper states of sleep.

If the body does not get enough deep sleep one day, it will compensate the next time it can get sleep by quickly moving through the cycles to reach the deepest levels of sleep more quickly and stay there longer.

However, if the person regularly does not get enough deep sleep, this may start to affect the brain.

As deep sleep plays a role in memory, an insufficient amount may lead to difficulty making new memories or retaining information.

Long-term issues with deep sleep may have an association with other conditions, such as heart disease or Alzheimer’s disease.

A person can take steps to increase the amount of deep sleep they get each night.

As the AASM notes, the most important thing that a person can do is to set aside more time for sleep. Doing so allows the body to go through more sleep cycles, which makes it possible to have more deep sleep.

Other practices that may help promote deep sleep, and good sleep in general, include:

  • doing vigorous exercise, such as swimming, jogging, or running, early in the day rather than before bedtime
  • making dietary changes that include eating fewer carbohydrates and more healthy fats
  • warming up the body in a spa or hot sauna before going to sleep

Pink noise may also increase the effectiveness of a person’s deep sleep. Pink noise is a random signal with more low frequency components than white noise. A study in the journal Frontiers in Human Neuroscience looked into the effects of sound stimulation, such as pink noise, on deep sleep. The findings indicated that listening to these sounds might enhance a person’s deep sleep state, leading to better memory function when they wake up.

Certain sleep habits might also help promote better sleep overall, including:

  • avoiding blue lights, such as smartphones or computers near bedtime
  • keeping the room as dark as possible by covering windows and turning off lights from alarm clocks and other electronic devices
  • avoiding caffeine later in the day
  • avoiding big meals before bedtime
  • practicing stress management techniques
  • setting a sleep schedule and trying to fall asleep and wake up at consistent times

Learn more about ways to sleep better.

Various factors can affect the quality of a person’s sleep. People can control certain factors, such as the time they go to bed and whether they stay awake looking at their phone or reading a book.

Other factors are more difficult to control. They include recognized sleep disorders and situations in which a person’s job or travel impairs the quality of their sleep. Factors that may cause a lack of deep sleep include:

Deep sleep has many functions, including replenishing energy, helping store memories, and balancing hormones.

The sleep cycle begins with light sleep, before leading to deep sleep and REM sleep.

Many factors can impair a person’s sleep quality, including an inappropriate sleep environment, eating or exercising too close to bedtime, and sleep disorders, such as sleep apnea and insomnia.

There may be some ways to promote deeper sleep, such as making certain dietary changes and listening to pink noise while falling asleep.

Getting more deep sleep may sometimes be as simple as setting aside more time to sleep each night.