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It can be difficult to get enough sleep every day amidst the busy daily routine of work, housework, and childcare. Many people, especially during the week, probably don't get much sleep.
Therefore, this article introduces the causes of shallow sleep and ways to improve it, for those who have trouble falling asleep, wake up many times during the night due to light sleep, or feel drowsy or fatigued upon waking.
Surprisingly many people experience shallow sleep?!
According to a survey on sleep quality conducted by the Ministry of Health, Labour and Welfare (※1), 21.8% of respondents, or about 1 in 5 people, answered that they were "not satisfied with their overall sleep quality."
The most common response was from women in their 30s, with 32.6% reporting dissatisfaction with their overall sleep quality. Considering that some people who did not report dissatisfaction may not be aware of their poor sleep quality, the actual number may be even higher.
※1: Ministry of Health, Labour and Welfare "2019 National Health and Nutrition Survey Report"
This problem of shallow sleep can sometimes be diagnosed as insomnia. For more details on insomnia, please refer to the following article.
It's morning and I couldn't sleep... Could it be insomnia?
What causes shallow sleep?
Consistently poor sleep quality can lead to a decline in concentration, memory, and judgment, affecting performance at work and in daily life, so caution is advised. However, in many cases, people don't know what is causing their poor sleep quality. Let's look at some common causes here.
Mental causes
Normally, the sympathetic nervous system is dominant during the day, and the parasympathetic nervous system is dominant during sleep. However, if the balance of the autonomic nervous system is disrupted by stress from work or daily life, the sympathetic nervous system becomes dominant even when trying to sleep, leading to insomnia and reducing sleep quality.
The secretion of a hormone called cortisol is also involved. Cortisol is a hormone that helps the body cope with stress, and it is also known to inhibit sleep. When stress is felt, the secretion of cortisol increases, leading to a decrease in sleep quality.
Conversely, if cortisol secretion decreases due to lack of sleep, for example, a person becomes more susceptible to the effects of stress during the day. This results in a vicious cycle where sleep becomes shallower and tolerance to stress decreases.
Physical causes
Frequent urination is a physical cause of shallow sleep. If you suffer from frequent urination due to aging or kidney dysfunction, you may wake up many times during the night with the urge to urinate. In addition, hormonal imbalances, including abnormal secretion of melatonin, the sleep hormone, and skin itching due to skin conditions can also disrupt sleep and reduce sleep quality.
Irregular lifestyle habits
There are many lifestyle habits that lead to a decrease in sleep quality, but recently, using smartphones and computers before bed has been particularly problematic. Exposure to blue light emitted from smartphone and computer screens before bedtime reduces melatonin, a hormone essential for high-quality sleep.
Furthermore, it weakens drowsiness and makes it difficult to fall asleep, so it is recommended to digitally detox 1 to 2 hours before bedtime.
Other causes of shallow sleep include long naps during the day and consuming alcohol or caffeine before bed. Conversely, a lifestyle habit to be mindful of is exposure to sunlight during the day. This promotes melatonin secretion at night, which can improve sleep quality.
Core body temperature not decreasing
The human body maintains a high temperature during the day for activity, but during sleep, it lowers its core body temperature (the temperature inside the body) to induce sleep and allow the brain and body to rest properly. At this time, the body lowers its core temperature by releasing heat from the skin surface. However, depending on the bedding, heat can become trapped in the body, making it difficult to fall asleep and get good quality sleep.
Suboptimal sleep environment
An uncomfortable room temperature or humidity, too much light, or excessive noise can make it difficult to fall asleep smoothly, and even if you do fall asleep, it can cause you to wake up in the middle of the night. Furthermore, if your bedding doesn't suit your body, your body won't rest properly, hindering recovery from fatigue.
Characteristics of people with shallow sleep
Common tendencies observed in individuals with shallow sleep include feelings of fatigue and lethargy upon waking, and strong daytime sleepiness. If you frequently have nightmares where you get reprimanded at work, it might indicate accumulated stress during the day. Additionally, people with disrupted circadian rhythms or autonomic nervous system imbalances, or those with excessive drinking habits, are also prone to shallow sleep, so caution is advised.
Impact of shallow sleep
So, what kind of problems does shallow sleep cause in daily life? The main negative effects are explained below.
Accumulation of fatigue
The cells, muscles, and organs in the human body work tirelessly every day. Just as humans rest to recover from work fatigue, cells, muscles, and organs also heal their fatigue by resting through sleep. Conversely, if you consistently don't get enough sleep, you won't recover from fatigue, and it will accumulate. As a result, aging will also accelerate.
Decreased productivity
The brain also needs sufficient sleep. The brain, which works actively during the day, rests at night to organize and store memories. However, if sleep is shallow, the brain remains in a sleep-deprived state and cannot properly organize memories. As a result, learning ability decreases and attention span becomes scattered, leading to a decline in productivity.
Increased risk of illness
Sleep quality is related to immunity, and shallow sleep and lack of sleep can lower immunity. This increases the risk of infections and lifestyle-related diseases, negatively impacting health.
Lifestyle-related diseases whose risk increases due to reduced sleep quality include metabolic syndrome, diabetes, and high blood pressure, among others. Additionally, it increases the risk of heart and brain diseases.
Mechanism of sleep
Understanding the mechanism of sleep is crucial for improving sleep quality.
Two systems that form sleep
Human drowsiness is related to two systems: sleep drive and arousal force.
Sleep drive is the desire that accumulates due to fatigue during active periods and becomes stronger the longer one stays awake. It gradually decreases as one falls asleep and disappears after sufficient sleep.
Arousal is the force generated by the circadian clock's instructions to activate the sympathetic nervous system, the secretion of awakening hormones, and an increase in core body temperature. It overcomes sleep drive and promotes wakefulness.
Arousal is strongest during the day and weakens around bedtime when the sleep hormone melatonin is secreted. In this way, we humans live according to a mechanism where we are active during the day due to arousal and fall asleep at night due to sleep drive.
Two functions that determine sleep rhythm
The human sleep rhythm is determined by two functions: circadian rhythm and sleep homeostatic control mechanism.
Circadian rhythm is the body's internal clock, referring to the bodily function that regulates body temperature, hormone secretion, and more, in a cycle of approximately one day. The sleep homeostatic control mechanism is the function that controls the aforementioned sleep drive and arousal force.
These two functions working normally allow us humans to naturally maintain a daily rhythm of activity during the day and sleep at night.
Relationship between NREM and REM sleep
There are two types of sleep: NREM sleep (a state where both the brain and body are in deep sleep) and REM sleep (a state where the body is resting but the brain is active).
Although there are individual differences, generally, immediately after falling asleep, one enters NREM sleep. Then, approximately 90 minutes after falling asleep, one transitions to REM sleep. After that, NREM sleep and REM sleep alternate approximately every 90 minutes. This approximately 90-minute cycle repeats 4-5 times a night, with more deep NREM sleep in the first 3 hours of sleep, and more REM sleep as dawn approaches.
What is the "Golden 90 Minutes"?
Growth hormone, which plays important roles such as creating essential elements for the body like skin, bones, and muscles, promoting cell proliferation and normal metabolism, and anti-aging, is secreted in greater amounts when you sleep soundly during the "golden 90 minutes." This also improves the quality of subsequent sleep, thereby regulating the overall sleep cycle.
Conversely, if NREM sleep is shallow or the proportion of REM sleep is high, the brain will not be fully rested even with prolonged sleep. Furthermore, waking up frequently due to slight light or noise at night increases, making it difficult to feel well-rested upon waking.
While the relationship between the brain and sleep is still not entirely clear, it is known that patients with depression and schizophrenia experience REM sleep immediately after insufficient initial NREM sleep. From this, it is thought that the quality of the initial NREM sleep may be very important.
The relationship between shallow sleep and dreaming a lot
If you want to know if your sleep is shallow, one indicator is how frequently you dream. Dreams are said to occur more frequently during REM sleep, which is a shallow sleep state where the brain is active. Therefore, if you dream a lot while sleeping, it is highly likely that your sleep is shallow. If you dream every day and experience strong daytime sleepiness, you should focus on creating a lifestyle and environment that promotes better sleep quality.
5 ways to improve shallow sleep
Here are 5 ways to improve sleep quality. If you feel like your sleep is shallow or of poor quality, please give them a try.
Review your lifestyle habits
Reviewing your lifestyle is one of the easiest ways to improve sleep quality. In particular, it's important to stop looking at smartphone and computer screens 1 to 2 hours before going to bed.
It's tempting to check social media or messages, but bright light from LCD screens can trick your brain into thinking it's daytime, making it harder to fall asleep. Avoid looking at LCD screens and spend time in dim, warm-colored light to help you fall asleep more easily. Also, avoid living without sunlight and sleeping with the lights on, as these are said to suppress the secretion of melatonin, the sleep hormone.
Review your eating habits
Eating habits are also an important factor related to sleep quality. For example, avoid drinking alcohol or caffeinated beverages such as coffee before bed, as these can hinder deep sleep.
Regardless of what you eat, eating immediately before bed is also a no-go. If you eat before bed, your stomach will continue to work to digest food even while you're sleeping, preventing your brain from getting enough rest. Finish your meal 2 to 3 hours before bed.
Review your bathing habits
Your core body temperature has a property where it drops as much as it rises. So, take a lukewarm bath of 38-40 degrees Celsius 90 minutes before bedtime to temporarily raise your body temperature and regulate it to drop during sleep. Lukewarm water activates the parasympathetic nervous system, allowing you to relax both mentally and physically and fall asleep more easily.
Also, when your body warms up and peripheral blood vessels dilate, heat is dissipated smoothly from your hands and feet. Therefore, generally, it is better not to wear socks when going to bed to achieve deeper sleep and improve sleep quality.
On the other hand, soaking in hot water above 42 degrees Celsius activates the sympathetic nervous system, making it difficult to fall asleep. While hot baths can make you feel refreshed and relieved of fatigue, they are not recommended for bathing before sleep.
Review room temperature and humidity
Checking the room temperature and humidity in the bedroom is crucial in summer and winter. In summer, a room temperature of around 26 degrees Celsius is considered ideal, and exceeding 28 degrees Celsius can decrease sleep quality. In winter, temperatures between 16 and 19 degrees Celsius are said to promote stable sleep. Also, the ideal humidity is 50-60%.
Review your pillow
Using a pillow that doesn't fit your body can lead to reduced sleep quality due to difficulty turning over, discomfort, and other issues. Use a pillow that suits you.
When choosing a pillow, height and size are particularly important. The ideal height is one that supports your head at an angle of 10-15 degrees from your neck when lying on your back, and where your chin is 0-5 degrees lower than your forehead. For someone with an average build, a height of about 6 cm is a good guideline. The ideal size is a width of at least three times the width of your head (60 cm) and a depth of at least 35 cm.
In addition to these, there are also pillows designed for sleep and custom-made pillows. Try to find a pillow that will help you achieve the best possible sleep quality. For more information on how to choose a pillow to improve sleep quality, please refer to the following article.
Diagnose insomnia with the Athens Insomnia Scale (AIS)
Here's the Athens Insomnia Scale (AIS), developed by the WHO (World Health Organization) to assess the severity of insomnia. If you're concerned about insomnia, feel that your sleep is shallow and unsatisfying, or still feel tired after a long sleep, try these 8 self-check questions. For each question, select the option that describes what you've experienced at least 3 times a week over the past month, and then calculate your total score.
How did you do? If your total score is less than 4 points, it indicates that you are getting good quality sleep. If you scored 4-5 points, there is a slight possibility of insomnia, so if you are concerned, consult a specialist. If you scored 6 points or more, there is a high possibility of insomnia, so without overexerting yourself, consult a specialist.
Improve shallow sleep! Check out recommended items
Brain Sleep develops various bedding products tailored to our customers' sleep needs and concerns. Here, we introduce the features and highlights of our recommended items.
From shallow sleep to deep sleep
If your sleep is shallow, you won't feel adequately rested even after long hours of sleep, leading to reduced performance during the day. Don't ignore it as just "the usual"; investigate the causes, take measures, and achieve satisfying, deep sleep.
Furthermore, if your sleep does not improve even after reviewing your lifestyle habits and sleep environment, consider seeing a medical professional. There is a risk that a serious illness may be hidden or that it may worsen into a sleep disorder.
Sleep is an important habit deeply connected to health. To live a vibrant life every day, strive for a lifestyle that prioritizes improving sleep quality.
