Slow wave sleep, also called deep sleep, is an important stage in the sleep cycle that enables proper brain function and memory. While most adults are aware that they should aim for between 7 and 9 hours of sleep each night, the science of sleep is quite complex.
The two main categories of sleep are called rapid eye movement (REM) sleep and non-REM sleep, and each has important stages. Humans 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 both better sleep and more deep sleep each night, allowing a person to wake up feeling more rested and refreshed.
The body first cycles through the three non-REM stages of sleep:
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 body starts to slow its rhythms down. The heart rate and breathing rate slow down, and the eyes begin to relax. The muscles also relax but may occasionally twitch.
The brain unwinds along with the body. The brain waves start slowing down as brain activity and sensory stimulation decrease.
The second stage of non-REM sleep is another lighter stage of sleep that occurs as the body starts transitioning to deeper sleep. As the National Institute of Neurological Disorders and Stroke note, 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.
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 during this part of the sleep cycle. The muscles and eyes are also very relaxed, and the brain waves become even slower.
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 and final 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 heartbeat and breathing rate speed up.
The REM stage is also when most dreaming occurs. The brain temporarily paralyzes the arms and legs to prevent the body from acting out these dreams.
While a person needs all the stages of sleep, deep sleep is especially important for brain health and function. Deep sleep helps the brain create and store new memories and improves its ability to collect and recall information.
This stage of sleep also helps the brain rest and recover from a day of thinking, allowing it to replenish energy in the form of glucose for the next day.
Deep sleep also plays a role in keeping the hormones balanced. The pituitary gland secretes human growth hormone during this stage, which helps tissues in the body grow and regenerate cells.
Importantly, a person has to get enough deep sleep for these functions to take place. The amount of deep sleep that a person has will relate to how much overall sleep they get. Sleeping 7 to 9 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 faster 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, the body may have difficulty making new memories or retaining information if it does not get enough sleep.
There may be a few ways to increase the amount of deep sleep that a person gets each night.
As the American Sleep Association note, the most important thing that a person can do to increase the amount of deep sleep that they get each night 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 may help promote deep sleep and good sleep in general, such as:
- doing vigorous exercise, such as swimming, jogging, or running, early in the day rather than before bedtime
- making diet changes that include eating fewer carbohydrates and more healthful fats
- warming up the body in a spa or hot sauna
Additionally, some antidepressants may help people get deeper sleep, although this is not the case for everyone.
Pink noise may also increase the effectiveness of a person's deep sleep. Pink noise is random noise with more low-frequency components than white noise. A study in the journal Frontiers in Human Neuroscience looked into the effects of using 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.
Some general healthful sleep habits may 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 shutting windows and turning off lights from alarm clocks
- avoiding caffeine later in the day
- avoiding big meals before bedtime
- reducing stress
- setting a sleep schedule and trying to fall asleep at the same time each night
Deep sleep is an important part of the overall sleep process, but it is just one aspect of a good night's sleep. There may be some ways to promote deeper sleep, such as tiring the body through exercise or listening to pink noise while falling asleep.
The best way to get more deep sleep may be as simple as setting aside more time to sleep each night.