A good night’s sleep is essential to good health. Among the different stages of sleep, deep sleep is perceived as the most important in feeling well-rested. But is this assumption valid? Is deep sleep the most crucial stage in determining a good sleep? How many hours of deep sleep do we need a night?
First, we need to answer the question, what is deep sleep?
What is Deep Sleep?
Deep sleep is also known as slow-wave sleep (SWS). It is characterized by delta waves or extremely slow brain waves. This stage of sleep is crucial to feeling refreshed in the morning and is considered the most restorative sleep phase.
SWS is part of Non-REM sleep, which is crucial for our bodies’ vital function. These include the production of growth hormones, strengthening of the immune system, repairing and regrowth of body tissues, and memory consolidation.
Deep sleep occurs in more extended periods during the first third of the night, with adults spending an average of 10-20% of total sleep time in deep sleep.
How is our Sleep Structured?
Sleep is composed of a series of sleep cycles during which the brain moves through four different sleep stages. Typically, we cycle through the sleep stages around 4-6 times per night with each cycle lasting approximately 90 minutes.
Generally, sleep stages can be divided into two types of sleep: non-REM sleep and REM sleep, as named after their ocular features of non-rapid eye movement and rapid eye movement.
The ratio of these two types of sleep changes throughout the night. Non-REM sleep dominates the early part of the night while REM sleep episodes continuously increase in duration as the night progresses.
Non-REM sleep refers to sleep stages N1 to N3, which includes deep sleep. This structural organization is commonly referred to as sleep architecture and is studied using the electroencephalogram (EEG). This test can detect the electrical activity of our brains and translate it into wavy lines.
What Happens in Each Sleep Stage?
Over these sleep stages, our brain waves become higher in amplitude and lower in frequency. This transition also influences our body motility as we move more during light stages, or REM sleep, and less during deep sleep.
When we’re awake, our brain waves consist of both alpha and beta waves. As we close our eyes and try to fall asleep, we become drowsy through a predominant alpha rhythm.
The first stage of sleep, N1, is characterized by less than 50% alpha waves. These are replaced by low-amplitude mixed-frequency activity as we drift in and out of sleep.
We can still wake up quickly during this stage, but our eyes move more slowly, and muscle activity slows down. This cycle lasts just 1 to 5 minutes and makes up approximately 5% of our total sleep time.
During the second stage of sleep, N2, brain waves become sleep spindles. These are 1-2 seconds of up and down waves low in amplitude and high in frequency. This rhythm shields the brain from external noises. Aside from this, K-complexes also occur in the brain. These are single high negative waves followed by a high positive wave. These suppress external stimuli and aid in memory consolidation.
During this phase, your body starts to prepare for deep sleep as your body temperature and heart rate drop. This stage lasts approximately 25 minutes in the initial cycle and lengthens with each successive cycle, totalling around half of our overall sleep time.
This lighter stage of sleep is essential for memory processing and consolidation. It also plays a fundamental role in processing and automating movement sequences. Thus, athletes highly benefit from this sleep phase after their training sessions.
N3 (Deep Sleep)
The third stage of sleep is when we enter deep sleep and find it difficult to wake up. If we do, we are likely to feel groggy and mentally impaired for 30-60 minutes.
During this phase, blood is redirected from the brain to the muscles, allowing them to receive oxygen and nutrients. The body repairs and regrows its tissues, growth hormones are produced, the immune system is strengthened, and memory consolidation continues.
During REM sleep, our brain waves are extraordinarily active and similar to when we are awake. This stage is also called paradoxical sleep because our body is completely paralyzed but our brain appears awake. This phase typically starts ~90 minutes after falling asleep.
During this phase, concentrations of the anxiety-triggering chemical, noradrenaline, is shut off. At the same time, the emotion and memory-related structures of the brain, the amygdala and hippocampus, are reactivated. Their activity allows for the processing of our memories and emotions.
Dreaming also occurs during this stage, which enhances our creativity through innovative insights and problem-solving skills.
Is Deep Sleep the Most Important Stage?
In short – no.
It is wrong to claim that one sleep stage, such as deep sleep, is more important than another. This is because both non-REM and REM sleep offer essential benefits that neither can accomplish alone. The functions of the brain are restored by and depend upon sufficient sleep – including all stages.
During N2 sleep, memory refreshment and motor memory occur. This allows us to wake up with a refreshed capacity to learn new information and movement sequences. On the other hand, N2 and N3 are important for memory consolidation, ensuring that we remember the things we’ve learned.
New and old information is fused and blended during REM sleep, boosting creativity and problem-solving. This sleep stage is also crucial for our mental health as it re-processes upsetting memories in a stress-free state. In turn, it dissolves the emotional charge of painful experiences and heals emotional wounds.
How Much Deep Sleep Do I Need?
The amount of deep sleep you should get ranges from 1-2 hours for a healthy adult. It is recommended that young adults get 7 to 9 hours of total daily sleep and older adults get 7 to 8 hours of total daily sleep. 10-20% of this time is spent in the N3 or deep sleep stage.
How Do You Know How Much Deep Sleep You Are Getting?
Although we can roughly estimate the hours of deep sleep we get based on our total sleep time, there are scientific ways to get a more accurate measure. One option is through wearable sleep tracking devices that help monitor our sleep patterns. Different tracking devices are compact, relatively inexpensive, and able to measure various sleep parameters including sleep stages, heart rate, and oxygen consumption.
What Causes Decreased Deep Sleep?
Alcohol consumption is associated with chronic sleep disturbance and lessened sleep. While alcohol acts as a sedative and reduces the time needed to fall asleep, it has been shown to disrupt sleep, particularly N3 and REM sleep.
Late-night snacking and fat intake in the evening is another factor that can disrupt sleep. These behaviours have been correlated with adverse effects on sleep quality: sleep efficiency, the time needed to fall asleep, shortening of sleep stages, and increased night-time awakenings. Moreover, late-night snacking can also lead to indigestion and acid reflux, further worsening sleep quality. We recommend eating at least 2 hours before going to bed to provide your body ample time to digest the food.
What Happens When You Don’t Get Enough Deep Sleep?
Getting below the recommended dose of sleep can result in sleep debt. This is the difference between the amount of sleep we need and the sleep we get.
Misbehaviours like this can lead to severe short and long-term consequences including increased weight gain, predisposition to obesity, a suppressed immune system, and impaired cognitive and physical performance. Sleep deprivation may also lead to increased sensitivity, irritability, impatience, and moodiness. Furthermore, inadequate sleep can raise diseases including cancer, hypertension, coronary diseases, stroke, depression, and diabetes.
How to Get More Deep Sleep
While it can be challenging to get sufficient sleep every night, practising proper sleep hygiene can ensure more consistent sleep quality and longer deep sleep. Some strategies to increase deep sleep are through exercise and listening to binaural beats.
Exercise of at least 1 hour is scientifically proven to increase total sleep time and deep sleep. While this depends on an individual’s fitness, high energy expenditure depletes energy stores and encourages more restorative sleep.
Another technique is by listening to binaural beats. This soundwave phenomenon is proven to relax the brain. It works by combining two slightly different sound frequencies to create a single new frequency tone perception. One study has shown that listening to binaural beats during sleep can affect sleep duration, prolonging the N3 (deep sleep) stage.
Aside from these, building an environment that promotes proper sleep hygiene and healthy habits can create a conducive sleep routine.
Here are some quick tips on how to increase deep sleep:
- Stick to a sleep schedule
- Do not lie in bed awake
- Do not take naps after 3 pm
- Keep your bedroom dark, cool, and free from electronic devices
- Avoid caffeine, nicotine, and alcoholic drinks (especially before bed)
- Avoid large or heavy meals and beverages late in the evening
- If possible, avoid medicines that delay or disrupt your natural sleep pattern
- Invest in a high-quality mattress, pillows, and bed linens
From science, we can find that deep sleep is fundamental to bodily restoration and feeling refreshed in the morning. However, all other stages of sleep are just as important as well. Each sleep stage has its benefits, which is why it’s vital to ensure sufficient total sleep time through proper sleep hygiene.
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