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Did you know there are two types of sleep: "non-REM sleep" and "REM sleep"? Many of you are probably aware from TV and online news that these two sleep cycles offer various benefits, such as "physical recovery during sleep" and "memory consolidation."
This article will introduce the characteristics of these two sleep patterns, from the basic roles of "non-REM sleep" and "REM sleep" to their functional aspects and other effects on the body. Understanding the characteristics of these two types of sleep will be very helpful in establishing an optimal sleep routine for yourself.
1. What's the Difference Between "Non-REM Sleep" and "REM Sleep"?
While humans appear to be in the same state when sleeping—eyes closed, body still, lying down—in reality, the cycle from the start of non-REM sleep to the transition to REM sleep occurs in 90 to 120-minute periods, repeating 4 to 5 times over a single night.
Within this, the deepest sleep occurs in the first cycle of "non-REM sleep." This initial 90 minutes is often called the "golden 90 minutes."
During this initial deep sleep, the pituitary gland secretes "growth hormone." Growth hormone is responsible for promoting skeletal formation, increasing height, recovering from fatigue, and regulating hormone balance. By getting deep sleep and ensuring proper growth hormone secretion, the brain and body recover from fatigue and all cells are repaired while we sleep.
While we sleep with our eyes closed, the brain and body undergo more activity than you might imagine, repeating two different sleep activities: "non-REM sleep" and "REM sleep." Approximately 20% of sleep is REM sleep and 80% is non-REM sleep, but both "non-REM sleep" and "REM sleep" have their own roles, creating the vital life activity that is sleep.
While sleep duration varies from person to person, getting at least 6 hours of sleep and repeating the "non-REM sleep" and "REM sleep" cycles at least 4 times is considered sufficient sleep. Since "non-REM sleep" and "REM sleep" each have their own roles, I will explain them in detail below.
1-1. Non-REM Sleep
"Non-REM sleep" plays roles such as resting the brain, repairing bodily tissues through the secretion of growth hormone, and enhancing immune function. In addition, during non-REM sleep, the body also plays a role in consolidating learned motor skills and techniques, such as exercise or playing musical instruments. Many of you might already know this.
When there is temporary sleep deprivation, such as after an all-nighter, this "non-REM sleep" tends to occur more readily, allowing for deeper sleep. Many of you may have experienced sleeping better than usual the day after an all-nighter.
Non-REM sleep, during which the brain is at rest, can be divided into three stages by observing brain activity through electroencephalography (EEG), with the depth of sleep changing with each stage.
- Stage 1: Light sleep where one can be easily awakened by small noises.
- Stage 2: Shallow sleep, characterized by light snoring.
- Stage 3: Deep sleep where one does not awaken to voices or sounds.
*Sleep deepens from stage 1 to 3, and slow, large waveforms (delta waves) are observed in the brain activity.
When transitioning into deep non-REM sleep, the brain enters a resting state. However, the muscles throughout the body remain active, allowing for movements like turning over to aid in the recovery of fatigued areas.
This activity refreshes the brain and body, recovering from the fatigue accumulated during daily life. Depending on the extent of this recovery, you can feel refreshed and satisfied when you wake up in the morning.
Achieving the deepest possible "non-REM sleep" during the first 90 minutes of sleep, when the body enters deep sleep mode, can regulate subsequent sleep rhythms and improve overall sleep quality.
Remember that these initial "golden 90 minutes" are crucial for the maintenance of your body.
1-2. REM Sleep
After falling asleep, the first non-REM sleep is followed by "REM sleep."
During REM sleep, the eyes move rapidly, which is why it is abbreviated from "Rapid Eye Movement."
REM sleep is generally considered shallow sleep, but the brain remains active and dreams. At this time, muscles are relaxed, and the body is completely still. Even if there are noises or loud sounds, the body's mechanism to block external stimuli acts, so it's not particularly easy to wake up.
During "REM sleep," the brain is active and is known to be organizing thoughts and consolidating memories. Therefore, it is important for brain development and emotional stability, and it is especially crucial for children in their growth stage to experience adequate REM sleep.
As morning approaches, the duration of non-REM sleep and REM sleep reverses, with REM sleep becoming longer in the latter half of sleep. This means that to ensure sufficient REM sleep, which is essential for brain development, it is also necessary to get enough sleep for a longer duration.
And as morning approaches and REM sleep increases, the body warms up, allowing for a comfortable awakening with the sunlight.
1-3. Non-REM Sleep and REM Sleep Alternate in a Fixed Cycle
"Non-REM sleep" and "REM sleep" are both crucial for the brain and body. These two types of sleep alternate in cycles of 90-120 minutes, and maintaining a good balance between them leads to quality sleep.
This 90-120 minute cycle, in which these two types of sleep alternately repeat, is called the "ultradian rhythm," and it is a rhythm that humans have within their bodies. If this state becomes unstable, the quality of sleep can decline, potentially negatively affecting health.
Some people may think that "if I sleep deeply, a short amount of sleep is enough." However, short sleep can lead to a temporary recovery of the brain due to deep "non-REM sleep," but the amount of "REM sleep" will be reduced.
Reduced REM sleep can lead to decreased cognitive ability and emotional instability. It is important to ensure sufficient sleep time and to have sleep where "non-REM sleep" and "REM sleep" repeat cyclically in a balanced manner.
It is ideal to secure 6-8 hours of sleep to allow for 4-5 cycles of non-REM and REM sleep per night.
By getting a total of about 7 hours of sleep, you can recover from fatigue, organize your thoughts, and wake up feeling refreshed.
If various factors such as alcohol consumption before bed or stress due to exam anxiety overlap, it becomes difficult to achieve the initial deep non-REM sleep.
REM sleep, in particular, is easily affected by daily lifestyle rhythms.
To achieve quality sleep, aim for deep "non-REM sleep" at the beginning and sufficient "REM sleep" in the latter half of your sleep, by ensuring adequate sleep time. This way, you can strive to maintain an appropriate sleep cycle of non-REM and REM sleep.
2. What is the difference in the roles of non-REM sleep and REM sleep?
Non-REM sleep and REM sleep differ not only in sleep depth but also in the nature and state of the activities occurring within the body.
A major difference is that "non-REM sleep" rests the brain, secretes growth hormone, and has the effect of regulating bodily functions.
On the other hand, "REM sleep" rests the body while the brain is active, working to organize information obtained during daily life. It is also known that most dreams occur during "REM sleep."
- Non-REM sleep: Brain rest, growth hormone secretion
- REM sleep: Body rest, thought organization, memory consolidation
During sleep, the brain and body alternately rest in this way, organizing memories and managing physical condition from within.
2-1. Non-REM Sleep: Brain Rest and Growth Hormone Secretion
A major role of non-REM sleep is the state of brain rest and the function of secreting growth hormone. During this sleep, which significantly affects physical growth, the brain enters a resting mode, and the regulation of bodily functions and the repair of internal tissues take place.
In addition, it plays crucial roles in maintaining vital functions such as skeletal formation, metabolic regulation including fat breakdown, and enhancement of immune function. Also, growth hormone is a hormone involved in growth, such as increasing height, but it is actually secreted from childhood to adulthood.
Although the secretion tends to decrease with age, it is still secreted in older individuals and plays a role in cell repair and metabolic regulation, which is why it is sometimes called an anti-aging hormone.
It is known that this growth hormone is secreted during deep non-REM sleep in the initial period of sleep, so it is ideal to enter deep sleep during the "golden 90 minutes" of initial sleep.
If this is achieved, growth hormone will be properly secreted, and children will grow, and adults will be able to maintain a body that is less prone to aging.
2-2. REM Sleep: Organizing Information and Consolidating Memories!
One of the important functions of sleep is memory consolidation. This is supported by REM sleep.
It has the role of organizing a lot of information and events acquired during the day as memories, and it works to consolidate what was learned from daily studies or work into memory.
It is also known that during REM sleep, the hippocampus, which is involved in learning and memory, becomes active, leading to the integration and organization of vast amounts of information, thereby consolidating memories while sleeping.
If sleep is insufficient, REM sleep also shortens, making it difficult to consolidate acquired knowledge and skills into memory. Quality sleep is essential for improving memory.
In fact, a study on REM sleep and memory conducted by McGill University in Canada using mice reported that when brain activity was interfered with during REM sleep, the content learned the previous day was less likely to be consolidated. *1
This means that if some stimulation causes awakening during REM sleep, the skills and knowledge acquired over time could potentially be lost.
3. How to Distinguish Between Non-REM and REM Sleep
So far, we have explained that non-REM sleep and REM sleep have different characteristics and roles. You might still wonder how to actually distinguish between them, even after understanding both functions.
During REM sleep, muscles relax and the body becomes still, so when someone is completely still and appears to be in a deep sleep, it's likely REM sleep. Additionally, during REM sleep, irregularities in pulse and breathing have been reported, which can serve as an indicator.
On the other hand, during non-REM sleep, the body moves moderately, so when there are moderate body movements, such as tossing and turning, it is highly likely to be non-REM sleep.
One way to distinguish them through bodily sensation is whether you remember a dream before waking up. While dreams are said to occur in both REM and non-REM sleep states, it is generally believed that if you wake up during REM sleep, the mental imagery remains in your memory.
Furthermore, a good morning awakening is said to occur during the latter part of the REM sleep cycle, so if you feel "refreshed upon waking," that is REM sleep.
On the other hand, if you are awakened during Stage 2 or 3 of non-REM sleep, you will be in a deep sleep, so your consciousness will be hazy. The deeper the sleep, the less likely you are to wake up from surrounding noise or disturbances, so if you cannot be woken up, it is likely that you are in Stage 2 or 3 of non-REM sleep.
It is extremely difficult to reliably distinguish between REM sleep and non-REM sleep without electroencephalography, and what is happening inside the body cannot be easily observed visually from the outside. The degree of awakening and physical sensations upon waking should only be considered as a means of prediction.
4. Optimal Sleep Duration to Maximize Each Type of Sleep
Sleep is something that is familiar to everyone. Rather than simply considering sleep as one way to relieve fatigue, it must be seen as a habit essential for enriching daily life and maximizing work performance.
Therefore, it is necessary to review the entire process from before bedtime to waking up and consciously make changes to achieve quality sleep. Among these, the most important is "securing enough sleep time."
Sleep duration should not be too short or too long. While the optimal sleep duration varies from person to person, the ideal is to get approximately 7 hours of sleep in total. If you wake up feeling refreshed after your first waking cycle, that is a sign of good sleep.
Ensure sufficient sleep time by focusing not only on the initial deep "non-REM sleep" but also on the "REM sleep" that becomes more prevalent in the latter half of sleep. Try to regulate your daily rhythm and prioritize feeling well-rested and refreshed when you wake up.
Summary: Regulate your sleep rhythm for quality sleep
Did you understand that regulating your sleep rhythm and ensuring adequate sleep duration is effective in all aspects, such as allowing your brain and body to recover during sleep and preparing for activity the next morning?
Let's review this article one last time.
- "Non-REM sleep" is for brain rest and growth hormone secretion.
- "REM sleep," where the brain is active, improves memory, makes you prone to dreaming, rests the body, and allows for a refreshing awakening.
- One sleep cycle is approximately 90-120 minutes, and as sleep progresses into the latter half, REM sleep becomes longer than non-REM sleep.
While sleep cycles vary by individual and age, the cycle of non-REM and REM sleep is approximately 90 minutes. Of course, there are also individual differences, ranging from 70 to 120 minutes.
Take this opportunity to deepen your understanding of sleep and pay attention to the depth and length of your sleep to maintain an optimal sleep cycle.
[Reference]
*1 REM sleep plays a critical role in memory formation, study finds
