Deep sleep is one of the Non-REM stages of our sleep and it is veiled by mystery. Not much has been discovered about the effects of deep sleep – although we are aware of some of the physiological and psychological benefits, as well as the negative impact of deep sleep deprivation. However, there is still much to learn.
Deep sleep is immensely important for memory consolidation and tissue regeneration. Unfortunately, the amount of deep sleep we get as we age gets smaller. Being ‘delicate’ and evasive, reaching deep sleep can be prevented by a large number of factors, such as various drugs, alcohol, stress, and sleep-related disorders.
All of this doesn’t mean we have to go through life deep-sleep deprived – we just have to tune into our natural biorhythm and take care of our health, hours of sleep, as well as sleep quality.
What part of sleep is deep sleep?
Our sleep cycle is typically divided into four stages. These are – Stage 1 and Stage 2, which fall into the category of light sleep, followed by Stage 3, which falls into the category of deep sleep, and finally REM (rapid eye movement) sleep. The first three stages together make up the Non-REM sleep, as opposed to REM sleep, and are consecutively labeled as N1, N2, and N3.
Deep sleep is also known as slow-wave sleep (SWS) and delta sleep due to the type of brain waves produced during sleep. Delta waves of low frequency characterize the N3 stage.
Are there four or five sleep stages?
There are four sleep stages, but they used to be divided a bit differently in the past. Deep sleep was considered to consist of two stages – 3 and 4. This division was made due to the frequency of delta wave occurrence – Stage 3 had a low amount of delta waves present whereas they were more frequent in Stage 4. This led scientists into conclusion that Stage 4 meant that the person had reached a deeper sleep. However, since 2008 the American Academy of Sleep Medicine refers to both of these as Stage 3 as there has been no clear evidence that there were two distinct stages of deep sleep.
When does deep sleep occur and how long does it last?
Deep sleep begins at about the 45th minute after falling asleep, that is, after our brain has cycled through Stages 1 and 2. It lasts significantly longer in the first half of the night than in the second. To be precise, we get the most of our nightly deep sleep in the first two sleep cycles.
A healthy adult has about 20% of deep sleep per night. However, the percent of the deep sleep we get depends on our age, sex, health, lifestyle and sleeping habits.
What happens during deep sleep?
Here are the physical, psychological and physiological processes in our body which occur during deep sleep:
- The release of growth hormone (GH), which helps a young body grow and develop and is still important in older ages for cell restoration and rejuvenation and tissue reparation
- Memory consolidation and transfer from short-term into long-term memory
- No response to external stimuli – a person may be very difficult to wake up from deep sleep; if woken, they may be disoriented and feel sluggish
- The immune system is boosted.
- Muscles are relaxed, no eye movement.
- Breathing slows down and heart rate and body temperature drop.
- Energy is restored for the following day.
- The following parasomnias occur – bedwetting, sleepwalking, night terrors.
- Glucose levels in the brain increase.
- Decreased metabolism allows oxygen byproducts to clear our of the brain.
What are the benefits of deep sleep?
Here are some of the positive aspects of getting enough deep sleep:
- Feeling rested. After having enough deep sleep, you should feel no need to sleep again any time soon. However, if you take a daytime nap long enough to reach deep sleep, your overall sleep propensity will decrease and you might end up going to bed very late.
- Your mind is refreshed and you will be able to learn new things easily.
- The tissues in your body are repaired and cells are restored. This is very important especially if your body is fighting a disease or an illness.
- The brain is cleared of plaque buildup
Deep sleep and age
Slow-wave sleep decreases as we age. A study has observed how the amount of delta sleep is associated with growth hormone and age in men. What was found is that the amount of delta sleep in young individuals (16-25 years of age) decreased from about 20% to 3.4% in mid-life (36-50 years of age).
Being associated with the release of growth hormone, deep sleep seems to be an indicator of youth.
Disorders linked to deep sleep
There are some sleep disorders (parasomnias) which typically occur during the N3 stage of sleep. They are also called Non-rapid eye movement sleep arousal disorders because a person is partially awake – enough to perform complex activities, but not enough to be considered fully awake. Bedwetting is not a sleep arousal disorder.
- Sleepwalking (somnambulism). Although more common in children, sleepwalking occurs with adults, too. It may be an indicator of sleep deprivation, intoxication, or medications. Good sleep hygiene often eliminates this problem.
- Night terrors or sleep terrors. During a night terror episode, people may seem awake as they express inconsolable panic, scream, hit or try to escape. They should be no reason for worry.
- Bedwetting (nocturnal enuresis). Mostly happens in children before the age of 7. It should not be considered a problem until this age. Bedwetting is mainly a sign of immaturity and stops when children learn how to control their bladder.
- Sleep sex (sexsomnia). This should not be confused with erotic dreams. Sexsomnia is an unusual behavior in which a person engages in sex acts while sleeping.
- Sleepwalking, night terrors and bedwetting happen more frequently in those who suffer from sleep apnea. Bedwetting is usually accompanied with sleepwalking and even night terrors. These disorders tend to run in families.
How to get enough deep sleep
In general, getting enough deep sleep equals a good night’s sleep. You should go to bed before midnight in order to get enough of deep, restorative sleep. However, an interesting phenomenon has been reported – not only do we get more deep sleep in the first couple of hours after falling asleep, but we also get more deep sleep early in the night. Therefore, if you go to sleep at 11 pm, you will get more deep sleep and more REM sleep. But if you go to sleep at 3 am, you are likely to have more REM sleep only.
So make sure to follow all of the good sleep hygiene rules, including these:
- Respect your circadian rhythm. Get plenty of sleep every night by keeping a regular sleep schedule.
- Avoid alcohol and nicotine. Don’t drink any amounts of alcohol to ensure you get your delta sleep.
- Seek help for sleep disorders. People who suffer from obstructive sleep apnea stop breathing several times a night, which causes frequent wakings and disrupts a normal sleep cycle. The more you wake up, the less deep sleep you get. Using a CPAP machine to treat sleep apnea may help.
- Avoid nightly wakings. Make your sleep environment calm and quiet. Turn your phone off and don’t drink liquids which may cause frequent toilet visits.
- Relax. Avoiding stress can positively affect your sleep – especially reaching deeper stages of it.
- Exercise for at least 30 minutes every day. Working out is believed to promote deep sleep because, after strenuous exercise, the body has to release growth hormone to support muscle growth.
After learning about its benefits, many people wonder how to increase deep sleep. Although what it means is sticking to habits which promote a good night’s rest, there are some other ways to increase deep sleep. One is the drug called gamma hydroxybutyrate (GHB), used by bodybuilders in the past because it supports muscle growth. After being misused as a ‘date rape drug’, it became illegal and is now used for treating narcolepsy and cataplexy.
Some researchers have discovered that playing certain soundwaves to sleeping individuals resulted in their brain responding by emitting low-frequency delta waves. This means that certain sounds made people reach deeper stages of sleep.
What is deep sleep rebound?
After a period of slow-wave sleep deprivation, the following night the brain will experience more SWS, thus making up for the lost deep sleep. A similar and more well-known phenomenon is REM sleep rebound in which a similar thing happens – the brain ‘hurries up’ to cycle through other stages to reach REM.
What are the differences between REM sleep and deep sleep?
Although frequently mistaken one for another, REM and deep sleep are not the same. This table shows the main differences (and similarities) between these two stages of sleep.
Non-rapid eye movement stage 3 sleep
Rapid eye movement sleep
Deep sleep, N3, stage 3, delta sleep, slow-wave sleep (SWS), orthodox sleep, synchronized sleep, (previously also stage 4)
REM, REMS, paradoxical sleep, desynchronized sleep, stage 4 (previously stage 5)
Occurs after stage
Stage 2 (N1, N2, N3)
Stage 2 (N1, N2, N3, N2, REM)
Occurs (minutes after falling asleep)
The longest periods of this stage happen
In the first half of the night
In the second half of the night
Faster than other sleep stages
Heart rate and breathing
Muscles relaxed, normally no movement
Muscles are paralyzed (muscle atonia), eyes move rapidly from side to side
May occur, not vivid
Very vivid dreams
Memory consolidation, good memory
body cell and tissue restoration, boosting of immune system
Memory consolidation, good memory
Better ability to read other people’s emotions, high emotional response to stress
Parasomnias, sleep disorders
Bedwetting, night terrors, sleepwalking, sexsomnia
REM sleep behavior disorder (RBD), narcolepsy, sleep paralysis
Deprivation from this particular stage may cause
Insulin resistance, low learning capacity, higher risk of Alzheimer’s
slow physical response, immune system weakening
Insulin resistance, low learning capacity, higher risk of Alzheimer’s
irritability, reduced coping mechanisms, migraines, higher risk of anxiety
Response to external stimuli
No response – a person is completely unaware of their surroundings, almost impossible to wake up
Dreams may depend on external stimuli, a person is easily woken up
After being abruptly woken up from this stage
A person is disoriented, not able to focus
A person reports being woken up from a dream
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