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You often hear that "sleep is not about quantity, but quality."
Sleep quality refers to "deep sleep and an ideal sleep cycle."
However, some people may not understand what deep sleep and an ideal sleep cycle are, or they may experience only shallow sleep and want to know how to get deep sleep.
Therefore, this article will explain deep sleep, an ideal sleep cycle, the importance of getting deep sleep, and introduce methods to achieve it.
By reading this article, you'll also understand how everyday habits can affect your sleep!
1. What are deep sleep and an ideal sleep cycle?
First, let's explain in detail the difference between deep sleep and shallow sleep.
Throughout one night's sleep, humans cycle through NREM and REM sleep 4 to 5 times. The number of these cycles and the depth of sleep determine sleep quality.
For example, even if you sleep for a long time, if you don't get enough deep sleep, you're likely to feel "not well-rested" or "tired despite having slept a lot." If you can only get a short amount of sleep, getting as much deep sleep as possible will allow your body to recover efficiently from fatigue.
1-1. Big differences between "REM sleep" and "NREM sleep"
Medically, the state where the body is asleep but the brain is active is called "REM sleep." This name is an acronym for "Rapid Eye Movement."
During REM sleep, even without consciousness, rapid eye movements occur. During REM sleep, the body is asleep, but the brain is awake. This is when dreams occur, and memory and emotions are organized.
On the other hand, sleep where both the body and brain are asleep is called "NREM sleep." Unlike REM sleep, rapid eye movements do not occur, hence the name.
NREM sleep is divided into three stages. The deepest sleep (stage 3) is said to occur within about three hours after falling asleep (*1). When in deep NREM sleep, the cerebrum, along with the body, rests its activity. Then, "growth hormone" is secreted. This hormone not only promotes physical growth but also aids in recovery from fatigue and repairs damaged cells.
In other words, even with a short amount of sleep, if you can enter deep sleep, you are more likely to feel satisfied and well-rested.
1-2. Sleep patterns vary from person to person
According to a 2016 survey by the Ministry of Internal Affairs and Communications Statistics Bureau, the average sleep duration for Japanese people aged 10 and over, including weekends, was 7 hours and 42 minutes.
On the other hand, a 2021 OECD (Organisation for Economic Co-operation and Development) survey found that the average sleep duration for Japanese people was 7 hours and 22 minutes, ranking last among the 30 member countries. Looking solely at these results, it might seem that Japanese people are sleep-deprived and need more sleep.
However, some people say, "I don't sleep for 7 hours, but I wake up feeling refreshed every day," while others say, "Even if I sleep for 8 hours every day, I wake up poorly and want more sleep."
In other words, unless it's extremely long or short, there's no need to be overly concerned about sleep duration. The Japan Medical Association also announced that "optimal sleep duration varies from person to person."
Furthermore, according to e-Health Net, operated by the Ministry of Health, Labour and Welfare, sleep duration tends to shorten with age. *2 Even people who worry that they "wake up early in the morning as they get older" do not need to worry excessively unless it interferes with their daily life, such as being overcome by unbearable drowsiness during the day.
If you sleep for a short period but wake up refreshed every day and don't experience strong drowsiness during the day, it's likely that you naturally require less sleep or have good sleep quality.
Conversely, if you sleep for more than 7 hours but struggle to get up, wake up feeling unrefreshed, or experience intense daytime drowsiness, it suggests that your body either requires more sleep than you're getting, or your sleep quality is poor.
2. What percentage of people actually get enough deep sleep?
According to a 2019 survey on sleep satisfaction targeting men and women in their teens to 70s, the percentage of people dissatisfied with their sleep was between 20% and 30% across all age groups.
Among them, men aged 25 and older had the highest percentage of dissatisfaction with sleep, with 29.5% of the total feeling dissatisfied. *3
However, it doesn't mean that the remaining 70% are satisfied with their sleep.
The survey included a category for "neither satisfied nor dissatisfied," and this was the most common response across all age groups.
Excluding "neither satisfied nor dissatisfied" and "satisfied" from the survey results, the percentage of people who responded "satisfied with my sleep" was around 37% to 42%.
In other words, including those who answered "neither satisfied nor dissatisfied," approximately half of all people in Japan across all age groups from their teens to 70s could be said to be "dissatisfied with their sleep." There is a close relationship between sleep satisfaction and deep sleep.
As mentioned earlier, if deep sleep is insufficient, it is difficult to feel "well-rested" even with long hours of sleep.
From the results of this survey, it can be said that only about half of the people are able to get enough deep sleep.
3. Not getting enough deep sleep can lead to many dangers.
Even if you sleep a lot, if it's only shallow sleep, you're unlikely to feel the satisfaction of "sleeping well," and you might even be sleep-deprived.
If you can't fall into deep sleep, you'll experience various problems such as waking up multiple times during the night, feeling intense drowsiness during the day, and having a melancholic mood.
However, many people may not notice this if they are getting a certain amount of sleep. Also, research shows that sleep deprivation tends to lead to obesity. *4
When sleep duration shortens, the secretion of "leptin," a hormone that suppresses appetite, decreases, and the secretion of "ghrelin," an appetite-stimulating hormone, increases.
In other words, continuous sleep deprivation can disrupt the secretion of appetite-related hormones, leading to obesity.
Obesity is a cause of lifestyle diseases such as "diabetes" and "high blood pressure." Diabetes and high blood pressure can also trigger diseases such as myocardial infarction, cerebral hemorrhage, and cerebral infarction, so caution is advised.
When obesity leads to fat accumulation around the tongue and throat, the airway can be compressed during sleep, leading to breathing cessation, often resulting in "sleep apnea syndrome." Without proper treatment, sufficient sleep cannot be obtained, and there is a possibility of accidents.
In fact, the JR Sanyo Shinkansen sleepy driving accident in 2003 and the Kan-Etsu Expressway highway bus sleepy driving accident in 2012 were both attributed to the respective drivers being diagnosed with "sleep apnea syndrome." *5
4. What to do to increase the proportion of deep sleep
So, what can be done to improve sleep quality and increase the proportion of deep sleep? Here are some recommended methods to increase deep sleep. Start with what you can.
4-1. Take a proper bath
Bathing promotes blood circulation and has a relaxing effect. Even in summer, warming your body after being chilled by air conditioning will make it easier to sleep soundly.
It is recommended to take a long bath in slightly warm water at 39-40℃ 1-2 hours before going to bed. This activates the "parasympathetic nervous system," also known as the rest nerve, and temporarily raises the body's core temperature. It then begins to decrease about 90 minutes later.
People feel sleepy when their core body temperature drops, so it is best to bathe 90 minutes before going to bed. Using your favorite relaxing bath salts is also recommended.
4-2. Use indirect lighting
If you stay in a bright place for too long, your brain will mistake it for morning.
If you experience difficulty falling asleep or shallow sleep, try dimming the lights and using indirect lighting for about an hour before going to bed. This will help you feel naturally sleepy and transition smoothly from light to deep sleep.
4-3. Avoid using your smartphone
Part of the light emitted from smartphones is "blue light."
This blue light has a wavelength similar to morning sunlight, and prolonged exposure can make the brain mistake it for morning, often leading to a loss of drowsiness.
Many people have experienced their eyes becoming wide awake after looking at their smartphone in bed.
If you go to sleep after using your smartphone, your sleep quality will suffer. The same applies to computers and portable game consoles. If you want to increase the proportion of deep sleep, stop using your smartphone or similar devices two hours before going to bed.
4-4. Make an effort to relax
Drowsiness is more likely to occur when you are relaxed.
Take a long bath, spend time in a room with quiet music, or do some light stretching in bed. Make an effort to relax yourself.
Just as there is a concept of a "sleep ritual," it's also recommended to find a method that helps you fall asleep easily. For example, cuddling a soft plush toy or diffusing your favorite aromatherapy oil.
However, avoid alcohol. While alcohol may induce drowsiness, it makes sleep shallower and actually reduces sleep quality.
4-5. Smokers should avoid smoking
Smoking activates the secretion of adrenaline. Adrenaline has the effect of activating the sympathetic nervous system, and also has awakening and blood pressure-raising effects.
For those who want to sleep soundly, smoking is counterproductive. Even if you feel "smoking is the ultimate relaxation," try to endure it and find other ways to relax.
Summary: Let's increase the proportion of deep sleep
There are many benefits to getting deep sleep.
While sleep duration is important, it's not just about getting enough hours.
Let's summarize the content of this article.
- There are two types of sleep: NREM sleep and REM sleep.
- To improve sleep quality, it's important to enter deep sleep during NREM sleep.
- There's no need to be overly concerned about sleep duration.
- Let's try methods to increase the proportion of deep sleep.
If you're struggling with "feeling sleepy even after sleeping a lot," please try the methods to increase the proportion of deep sleep.
[References]
※1 Ministry of Health, Labour and Welfare Health Information Site for Lifestyle Disease Prevention
※2 Ministry of Health, Labour and Welfare Health Information Site for Lifestyle Disease Prevention
※3 29th Lifestyle Health Basic Survey ~AI Analysis! "Sleep Satisfaction" for Ages 10-70~
※4 Sleep Deprivation is the Enemy of Diet
※5 Ministry of Land, Infrastructure, Transport and Tourism Response to Sleep Apnea Syndrome (SAS) Issues
