Like eating and exercising, sleep is crucial for your health. There are four sleep stages, and each stage has a specific purpose. Deep sleep is the repairing and restorative sleep that is necessary to feel your best each day.
Because of the potential impacts of obtaining insufficient deep sleep, it is important to understand what deep sleep is, the benefits of deep sleep, and the signs of insufficient deep sleep.
What Are the Stages of Sleep?
Once you fall asleep, your body cycles through three non rapid eye movement (NREM) phases of sleep followed by one rapid eye movement (REM) sleep stage. It usually takes between 90 and 120 minutes to cycle through all four stages, after which the cycle starts again. Adults typically have 4-6 such cycles per night. In the first half of the night you spend more time in NREM sleep. However, as the night goes on, you spend more and more time in REM sleep.
- Stage 1: This brief, drowsy stage marks the transition to sleep, when your breathing and heartbeat slow down.
- Stage 2: In this stage of light sleep your breathing and heart rate slow even more. Your temperature drops and your muscles relax. Stage 2 sleep lasts longer in each cycle throughout the night. About half of your total sleep every night is spent in this stage
- Stage 3: Previously divided into stages 3 and 4, stage 3 slow-wave sleep represents the deepest sleep of the sleep cycle, when brain waves are at their slowest in frequency and highest in amplitude
- REM: As the name suggests, your eyes move quickly beneath your eyelids during REM sleep. Your brain activity is similar to that of a person who is awake. However, you lack muscle tone and do not move. Experts believe most of our dreaming happens during REM sleep.
What Is Deep Sleep?
Deep sleep, also called slow-wave sleep, occurs in the third stage of non-rapid eye movement (NREM) sleep. During deep sleep, electrical activity in the brain appears in long, slow waves called delta waves. These waves have a frequency of 0.5 to 2 Hertz and they must make up at least 6 seconds of a 30-second window for that window to count as deep sleep.
Typically you descend into deep sleep within an hour of falling asleep, and experience progressively shorter periods of deep sleep as the night wears on. During this stage, automatic body functions like breathing and heart rate are also very slow and your muscles are relaxed. It can be difficult for someone to wake you up, and waking up out of deep sleep may make you feel mentally foggy for up to an hour.
Why Is Deep Sleep Important?
While all stages of sleep are necessary for good health, deep sleep offers specific physical and mental benefits. During deep sleep, your body releases growth hormone and works to build and repair muscles, bones, and tissue, and immune system functioning. Additionally, slow-wave sleep may be important for regulating glucose metabolism. Elite athletes value slow-wave sleep as it helps replenish energy stores.
Deep sleep is important for cognitive function and memory, and it is thought to play a role in language learning, motor skills, and the developing brain.
Throughout the day, you receive information inputs that strengthen the synapses, or points of communication, in the networks of your brain. However, your brain cannot take on information forever without rest.
Researchers suggest that deep sleep plays a role in preparing your synapses for the next day. Put another way, your brain evaluates new memories and then preserves and consolidates only the ones that are most relevant to avoid saturating memory pathways. Although this theory is still under investigation, evidence shows that people obtain a higher proportion of deep sleep after learning a new task, and show higher concentrations of slow waves in brain areas related to the task.
The results of this process are evident: after a night of good sleep, you are better prepared to take on new information and adapt to new environments.
How Much Deep Sleep Do You Need?
To calculate how much deep sleep you need, first determine how much sleep you need overall. Most adults should aim for seven to nine hours of sleep each night. Between 13% and 23% of that time should be spent in deep sleep. If you get seven hours of sleep each night, then you spend approximately 55 to 97 minutes each night in deep sleep.
To a certain extent, the body self-regulates amounts of deep sleep. For example, you might spend more time in deep sleep if you are recovering from sleep deprivation or if you regularly experience short sleep, such as over the course of a work week. People may also experience more deep sleep when sleep disorders such as obstructive sleep apnea are treated. By contrast, people who nap frequently may experience less deep sleep during subsequent naps, as part of their deep sleep needs have already been fulfilled.
As people age, they tend to have less deep sleep. They usually get more stage 2 sleep instead.
Sleep Disorders Associated with Deep Sleep
Sleep disorders that are specifically linked to deep sleep are called disorders of arousal and include sleepwalking, sleep terrors, and confusional arousals. While adults can experience these disorders, they are more common in children and adolescents.
Typically episodes of these disorders are short and the sleeper does not remember them. However, the events can impact your waking hours. Some sleepers with disorders of arousal experience excessive daytime sleepiness. Others may accidentally injure themselves or others during an arousal episode.
Measurements of the brain waves of sleepwalkers have found that many individuals continue to experience a certain proportion of slow waves — which are characteristic of deep sleep — during a sleepwalking episode. Adult sleepwalkers also display variances in slow-wave activity during the rest of the night.
What Happens When You Don’t Get Enough Deep Sleep?
In addition to causing feelings of fatigue, a lack of deep sleep can have a number of impacts on your body.
Because deep sleep is part of the memory formation process, you may struggle to consolidate memories after nights without enough deep sleep. Even after one night of insufficient sleep you may experience difficulty learning or remembering information.
On a physical level, insufficient deep sleep may decrease your immune response to vaccines and leave you more vulnerable to infection. During deep sleep, potentially harmful waste products are eliminated from the brain. As a result, disruptions to deep sleep may also drive advancement of neurological diseases such as Alzheimer’s and Parkinson’s.
Additionally, sleep deprivation is associated with hormonal changes that drive our appetite for high-calorie food. Going short on slow-wave sleep, in particular, is believed to contribute to insulin resistance and the development of type 2 diabetes and heart disease. Catching up on sleep may reverse some of these effects.
Who Doesn’t Get Enough Deep Sleep?
As deep sleep occurs in multiple stints throughout the sleep period, anyone who sleeps for less than the recommended amount of hours is likely to obtain insufficient deep sleep. Fragmented sleep due to sleep disorders or sleeping at times that are not concordant with your natural sleep-wake rhythm may also diminish the percentage of slow-wave sleep.
Some people with insomnia experience changes in their sleep cycles and as a result may have more stage 1 sleep and less deep sleep. Stress and aging can also reduce levels of deep sleep. Additionally, people with conditions such as schizophrenia and Alzheimer’s disease experience less slow wave sleep.
Recently, some researchers have observed that people with a damaged hippocampus, or memory center of the brain, experience much less slow wave sleep than people with an undamaged hippocampus. Experts propose that the memory signals sent by the hippocampus are a necessary trigger for creating the slow delta waves seen in deep sleep. Accordingly, problems with memory formation may represent a cause, rather than a consequence, of less deep sleep.
Signs You May Not Be Getting Enough Deep Sleep
Some indications you are not getting enough deep sleep include:
- Feeling unrefreshed and drowsy
- Reduced alertness and attention
- Trouble learning and forming new memories
- Cravings for high-calorie food
Tips for Getting More Deep Sleep
Ensuring you get sufficient sleep overall can help you get the deep sleep you need. By establishing consistent sleep and wake times, you can develop a healthy sleep routine for your body. Practicing good sleep hygiene can also help you get more sleep in total. Healthy sleep habits include:
- Exercising regularly
- Reducing caffeine intake in the afternoon and evening
- Ensuring you have a quiet, cool, and dark sleep environment
- Creating a relaxing routine to wind down in the evening
There are some additional steps you can try to encourage more deep sleep:
- Take a Warm Bath: Heating your body at least an hour before bedtime may help induce slow-wave sleep. The warmth from the bath draws heat to your hands and feet which then dissipates. The process allows you to cool down to a comfortable temperature for sleeping.
- Improve Your Diet: What you eat and drink before bedtime impacts your sleep. One small study found that people eating a diet high in saturated fats obtained less slow-wave sleep. People who ate more fiber were more likely to have more deep sleep.
- Listen to Binaural Beats: Binaural beats are created by listening to two slightly different tones, one in each ear. The difference between the frequencies of those tones creates a perceived third tone, or binaural beat. If certain frequencies are used, your brain waves can then be induced to match the frequency of that beat. Limited research suggests that listening to delta wave binaural beats may help induce delta waves in the brain and therefore stage 3 sleep.
When to Talk to Your Doctor
In addition to sleeping enough hours, restful sleep requires obtaining a proper balance of all the sleep stages. If you are experiencing sleep-related problems with your day-to-day mood, cognitive ability, performance, or physical health, talk to your healthcare provider. If possible, keep a sleep diary where you record your sleep habits and daytime experiences and share this information with your doctor.
Your doctor may have you take a sleep study or polysomnography. During a sleep study, a machine can record the electrical activity happening in your brain. These recordings show what stages of sleep you are in and for how long. The results of your study may provide a clearer picture of your sleeping patterns, which can help guide a treatment plan.
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