[Reading Science] The Unchanging Law That Causes Insomnia... Protect Your 'Kkuljam'

Modern Sleep Duration Decreases by 1 Hour Over 100 Years
Digital and Globalization Trends Worsen Sleep Quality
Declining Sleep Quality Linked to Disease and Mortality
Understand Individual Traits to Protect Sleep
"Keep Sleep Disruptors as Far Away as Possible"

In midsummer, high humidity and tropical nights are the biggest enemies of a “good night's sleep.” Especially modern people are sleeping less and less due to the use of digital devices, suffering from various side effects and even increasing the risk of premature death. Experts advise that it is desirable to understand individual patterns well and block out as many factors that interfere with sleep as possible.

Sleeping Worse and Worse

The quality of sleep significantly affects health, attention, memory, emotional regulation, work performance, and diseases. However, modern people's sleep is continuously decreasing. A 2017 study surveyed 690,000 children from 20 countries between 1905 and 2008 and found that sleep time decreased by more than one hour. As of 2019, about one-third of American adults were found to sleep less than six hours per day. A 2012 study of adults in seven countries including Japan, Russia, Finland, Germany, Belgium, and Australia revealed that from the 1960s to the 2000s, sleep time decreased annually by 0.1 to 0.6 minutes over more than 40 years. This is because many people spend more time on daily activities than on sleep due to digitalization and globalization. Especially, watching TV or using social networking services (SNS) on computers, laptops, and mobile devices greatly disrupts sleep. Office workers find it even harder to get comfortable sleep due to long working hours and working around the clock.

The Key to “Good Sleep”: Slow-Wave Sleep

How can we get quality sleep? Sleep was once considered a state similar to death and was largely ignored. However, in 1929, Hans Berger, a German psychiatrist, discovered specific brainwave patterns during sleep using electroencephalography (EEG), then a cutting-edge technology, which led to active research. Today, scientists generally classify sleep into two types: rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep is further divided into three stages based on brainwave patterns: N1, N2, and N3. Typically, people cycle through REM sleep, N1, N2, and N3 stages in about 90 to 100-minute intervals, repeating 4 to 6 times each night.

The human brain shows complex high-frequency activity when awake, with many active neurons. When just falling asleep or in the N1 stage, brainwaves shift to low-frequency “theta” waves, lasting about 1 to 10 minutes. In the N2 stage, body temperature drops, heart rate and breathing slow down, and muscles relax. Brainwaves also slow, with occasional short bursts. This stage usually lasts 10 to 25 minutes in the first sleep cycle and lengthens with longer sleep, accounting for about half of total sleep time. The next stage, N3, is slow-wave sleep, characterized by slow, high-amplitude delta waves. It occurs for about 20 to 40 minutes per cycle, roughly one-quarter of normal sleep time. This slow-wave sleep is the core of what is called “good sleep.” Mark Wu, a professor of neurology at Johns Hopkins University, emphasized in the international journal Nature on the 17th (local time) that “slow-wave sleep is the key element that makes people feel good after sleeping.” He explained, “Slow-wave sleep plays a crucial role in increasing energy storage, repairing and growing tissues in the body, clearing waste, and strengthening the immune system,” adding, “It also helps with learning and memory consolidation.”

REM sleep is also valuable to humans. Neural activity peaks, and dreaming occurs. REM sleep takes up about one-quarter of daily sleep time, with brainwaves forming beta waves. These are a mix of high and low frequencies, resembling the awake state. As the night deepens, slow-wave sleep duration shortens while REM sleep lengthens. REM sleep plays a key role in cognitive functions such as emotional regulation, memory, and creative problem-solving.

However, humanity still does not fully understand sleep. Professor Wu said, “Sleep is a phenomenon that occurs through the network activity of the entire brain, so understanding it precisely is very difficult,” and explained, “Among major human behaviors and physiological functions, sleep remains the only area whose function is not yet fully understood.”

Sleep Varies by Individual

Sleep is highly multidimensional. Sleep quality can be measured subjectively by how an individual feels, but also objectively by numbers such as duration, how long it takes to fall asleep after lying down, and how often one wakes up. Adults are generally advised to get 7 to 8 hours of sleep per day, but some people feel sufficient with only 3 hours, while others need 12 to 14 hours to feel satisfied. Sleep and wake times also vary. Some are early birds who go to bed and wake up early, while many are night owls who sleep and wake late. This is because each person’s circadian clock differs, and factors like homeostasis (internal regulation to sleep when sleepy and wake after sufficient sleep) and stress vary individually. When the circadian clock and actual sleep are not synchronized, sleep quality declines and fatigue increases. Night shift workers, people who sleep during the day, or those adjusting to jet lag are typical examples.

Keep Sleep Enemies as Far Away as Possible

The biggest enemy of sleep is the state of “arousal.” Arousal caused by coffee, cigarettes, excitement, or stress prolongs the time it takes to fall asleep. It also shortens sleep duration or causes fragmented sleep, reducing quality. Exposure to light especially suppresses the secretion of melatonin, the hormone that induces sleep. Blue light emitted from mobile devices, TVs, or energy-saving light bulbs is the worst. To improve sleep quality, it is better to exercise before bedtime rather than dozing off or napping. Moderate exercise for about 30 to 45 minutes a day is recommended instead of excessive exercise. Caffeine blocks adenosine, a hormone that regulates the sleep-wake cycle, from binding to receptors in the brain, so consuming it late in the afternoon is not advisable. Yoga or meditation can relieve physical and mental tension, increasing sleep time and helping chronic insomnia patients. For mild sleep disturbances, playing quiet music is beneficial. It prevents lying in bed worrying unnecessarily or falling into a hyperarousal state. However, audiobooks are not effective. Stress from family conflicts or looming deadlines is also an enemy of sleep. It increases the secretion of cortisol, a hormone in the body, disrupting sleep patterns.

Lack of sleep or poor sleep quality directly affects work and home life, causing various problems due to cognitive decline such as reduced attention. Chronic sleep deprivation is comparable to alcohol addiction. It can cause obesity, heart disease, diabetes, and lead to premature death. Professor Jilei Shan of the Nutrition and Epidemiology Department at Huazhong University of Science and Technology in China pointed out, “Sleep habits greatly influence lifestyle factors such as diet, physical activity, smoking, and drinking,” adding, “People who sleep less feel fatigued and thus do not exercise, while those who sleep too much or wake up late have no time to exercise or eat breakfast.”