A Scientific Approach to Improving Sleep Quality

What Is Sleep Quality?

Sleep quality is not simply a matter of sleeping longer. In sleep science, the indicators of sleep quality include the time it takes to fall asleep, the number of nighttime awakenings, the proportion of deep sleep (non-REM sleep stage 3), and how refreshed you feel on waking.

The "Sleep Guide for Health Promotion 2023" published by Japan's Ministry of Health, Labour and Welfare in February 2024 recommends that adults "secure the sleep they need, using six hours or more as a rough guide, while recognizing that the appropriate amount of sleep varies from person to person." The point is that there is no single correct number of hours that applies to everyone; it is a rough guide and nothing more. And even with the same seven hours of sleep, next-day cognitive function and mood differ greatly depending on whether that sleep was of high or low quality.

Creating an Environment for Better Sleep

Bedroom Temperature and Humidity

The room temperature for sleeping is generally put at around 18 to 20 degrees Celsius. The World Health Organization (WHO) housing guidelines recommend keeping indoor temperatures at 18 degrees or above in winter, and you want the bedroom not to fall below that either. The human body needs its core temperature to drop as it falls asleep, and a room that is too warm gets in the way of that drop, which makes falling asleep difficult. Humidity of 40 to 60% is considered comfortable.

Managing Light Exposure

The secretion of melatonin (the sleep hormone) is suppressed by light. From one to two hours before bed, it is recommended that you switch the room over to warm-toned indirect lighting and avoid the blue light of smartphones and computers.

In the morning, conversely, getting sunlight immediately after waking resets the body clock and promotes melatonin secretion at night. Even on cloudy days, outdoor light is several times brighter than indoor lighting, so the effect is more than sufficient.

Sound Environment

Sudden noise disturbs sleep, but there is research indicating that steady white noise or natural sounds can actually help you fall asleep. Earplugs are another effective option.

Reviewing Lifestyle Habits

Caffeine Intake Timing

The half-life of caffeine is roughly five to six hours. If you drink coffee at 2 PM, half of that caffeine is still in your body at 8 PM. To keep the impact on sleep to a minimum, it is best to avoid caffeine from the early afternoon onward.

The Effects of Alcohol

Alcohol does speed up the onset of sleep, but it reduces REM sleep in the second half of the night and increases nighttime awakenings. Using alcohol to fall asleep reliably lowers sleep quality, so it is recommended that you finish drinking at least three hours before bed.

Bathing Timing

With a bath one to two hours before bed, body temperature rises for a while and the drop that follows promotes sleep onset. Soaking for 15 to 20 minutes in lukewarm water of 38 to 40 degrees Celsius is effective. Water that is too hot stimulates the sympathetic nervous system and has the opposite effect.

Habits That Disrupt Sleep

Smartphone Use Before Bed

Beyond the blue light issue, scrolling through social media and news puts the brain into an alert state. Make it a habit to leave your smartphone outside the bedroom 30 minutes before you go to sleep.

Irregular Sleep Schedules

"Social jet lag," in which wake-up times differ by two hours or more between weekdays and days off, throws the body clock out of order and brings on a chronic decline in sleep quality. Ideally you would get up at the same time on days off as on weekdays.

Reading up on sleep science lets you understand your own sleep patterns more deeply.

When You See No Improvement

If sleep quality does not improve even after reviewing your lifestyle habits, a sleep disorder such as sleep apnea syndrome or restless legs syndrome may be lurking behind it. If insomnia continues for more than two weeks, consider a visit to a sleep clinic.

Using over-the-counter sleep supplements or sleeping pills on your own judgment is not recommended. Sleeping pills in particular carry a risk of dependence, so use them only under a doctor's guidance.

Making Use of Naps, and the Cautions

In a study reported in 1995 on flight crews on long-haul routes, crew members who took a planned short nap during the flight maintained their alertness and performance through the critical phases of descent and landing that followed. A short nap when you feel the afternoon slump is an effective way to supplement your nighttime sleep.

The key, though, is to keep naps to 30 minutes or less. Go past 30 minutes and you enter deep sleep, which leaves you groggy on waking, a state known as sleep inertia. Naps after 3 PM also get in the way of falling asleep at night, so avoid those. The "coffee nap," in which you drink a coffee before lying down, is another technique: you wake 20 minutes later, just as the caffeine starts to take effect, and come to cleanly.

Key Takeaways

• Learn the concrete steps for creating an environment for better sleep
• Get hold of the knack for reviewing lifestyle habits
• Let go of the habits that disrupt sleep
• Grasp how bedroom temperature and humidity work

Summary - Sleep Is an Investment

Improving sleep quality has a good effect on daytime performance, on mental health, and on long-term physical health alike. Making use of sleep-related goods also helps improve your sleep environment. Start by cutting back on smartphone use before bedtime.