睡眠とは？必要な睡眠時間・睡眠の質・体内時計と眠りを整える生活習慣を解説

What Is Sleep? Sleep Duration, Sleep Quality, Body Clock & Healthy Sleep Habits Explained

OBARASOTARO

“How many hours of sleep do I need?” “Is seven hours enough?” “Does good sleep quality simply mean getting more deep sleep?” “Can sleeping longer on weekends make up for short sleep during the week?” “How much do smartphones and coffee before bed really matter?”

Sleep is something we experience every day, but it is actually a complex physiological process involving the brain, body, circadian clock, light, body temperature and everyday habits.

You may have heard ideas such as “eight hours is the ideal amount of sleep” or “sleep runs in 90-minute cycles, so you should wake after a multiple of 90 minutes”.

Modern sleep science shows that sleep requirements vary between individuals and across different ages, and that sleep-cycle length also changes throughout the night.

When thinking about sleep, it is important to consider not only how long you sleep, but also whether you feel adequately rested by your sleep.

This guide explains what sleep is, REM and non-REM sleep, sleep cycles, sleep duration, sleep quality, the circadian clock and melatonin, morning and evening light, smartphones, exercise, bathing, caffeine, alcohol, naps, weekend sleep, sleep supplements, Japan's Foods with Function Claims system and sleep tracking.

Contents

What is sleep and why do we need it?

Even while we are asleep, the brain and body remain biologically active.

Sleep is associated with:

  • physical and mental rest
  • brain functions involved in memory and learning
  • metabolism and endocrine function
  • immune function
  • growth and development
  • cognitive function and attention the following day

Japan's Ministry of Health, Labour and Welfare also describes sleep as an essential form of rest for maintaining and promoting health in its Good Sleep Guide 2023.

A useful way to think about sleep is as a physiological period that helps support our ability to function while awake.

What makes us sleepy? Sleep pressure and the body clock

The tendency to become sleepy at night can be explained largely by two interacting systems.

1. Sleep pressure builds the longer we stay awake

As the number of hours since waking increases, the biological pressure to sleep gradually becomes stronger.

This is known as homeostatic sleep pressure.

Sleep pressure falls during adequate sleep and builds again as we remain awake.

2. The circadian clock provides timing information

The other major system is the circadian rhythm, which helps regulate the timing of sleep and wakefulness over an approximately 24-hour cycle.

The brain contains a central circadian clock that uses environmental signals, especially light, to align the body's rhythms with day and night.

Sleepiness is therefore influenced by:

“how long you have been awake” + “what time your internal clock thinks it is”.

Understanding these two processes makes it easier to see why when you sleep can matter as well as how long you sleep.

REM sleep and non-REM sleep

Human sleep is broadly divided into:

  • non-REM sleep (NREM sleep)
  • REM sleep

Non-REM sleep

Non-REM sleep is further divided into:

  • N1
  • N2
  • N3

N1 is relatively light sleep. N2 follows, and N3 is characterised by more slow-wave brain activity and is often called slow-wave sleep or deep non-REM sleep.

Deep non-REM sleep tends to be more prominent during the first part of the night.

REM sleep

REM stands for Rapid Eye Movement.

During REM sleep, parts of brain activity show features that resemble wakefulness, while activity in most skeletal muscles is strongly reduced.

REM sleep is also well known as a stage in which vivid dreaming is common.

Across the night, non-REM and REM sleep alternate. Deep non-REM sleep tends to be more common earlier in the night, while REM periods generally become longer toward morning.

Did you know? Is sleep really based on 90-minute cycles?

You may have heard the idea:

“Wake up after a multiple of 90 minutes and you will feel more refreshed.”

Non-REM and REM sleep do alternate in recurring cycles, and adult sleep cycles are commonly around 90 to 120 minutes.

A typical night's sleep may contain around four to six cycles.

However, cycle length varies between individuals and can also vary from one cycle to the next within the same night.

Research in healthy adults has found substantial variation in sleep-cycle duration.

For practical sleep planning, it is therefore more useful to consider your total sleep requirement and how you feel after waking rather than calculating bedtime purely from multiples of 90 minutes.

How much sleep do adults need?

For adults, a practical starting point is to think in terms of around six to eight hours while identifying the amount of sleep that suits you personally.

Japan's Good Sleep Guide 2023 states that for adults,

around six to eight hours represents the central range of appropriate sleep duration, with at least six hours generally encouraged.

Individual sleep requirements vary.

Some people feel adequately rested with a little over six hours, while others may need eight hours or more.

Useful factors to consider together include:

  • total sleep duration
  • how rested you feel on waking
  • daytime sleepiness
  • whether you can work, study, drive and function comfortably during the day

Focus on the sleep you need rather than simply extending time in bed

Spending substantially longer in bed than the amount of sleep your body needs can sometimes result in more time awake before falling asleep or more wakefulness during the night.

A useful goal is to secure enough sleep for your needs while keeping time in bed reasonably aligned with actual sleep.

Sleep needs change with age

Sleep requirements also change across the lifespan.

Japan's sleep guidance notes that the amount of sleep people are physiologically able to obtain tends to decrease gradually with age.

In adulthood, it is useful to understand age-related changes in sleep while identifying a sleep duration that suits your own needs.

Age group General guidance
Primary school children Use 9–12 hours as a general reference
Junior high and high school students Use 8–10 hours as a general reference
Adults Around 6–8 hours, with at least 6 hours as a general target while identifying individual needs
Older adults Consider both actual sleep duration and total time spent in bed while maintaining a regular daily rhythm

For older adults, sleep time and time in bed are different measures

As people age, physiologically required sleep may become shorter even while the amount of time spent in bed becomes longer.

Japanese sleep guidance uses keeping total time in bed under about eight hours as one general reference point for older adults while still ensuring adequate sleep.

Keeping daytime activity and night-time sleep clearly differentiated can help support an age-appropriate sleep routine.

How much sleep are people in Japan getting? 2024 National Health and Nutrition Survey

In Japan's 2024 National Health and Nutrition Survey, the most common average daily sleep duration among adults aged 20 and over was six to less than seven hours.

Overall:

  • Less than 5 hours: 7.2%
  • 5 to less than 6 hours: 30.4%
  • 6 to less than 7 hours: 34.9%
  • 7 to less than 8 hours: 19.2%
  • 8 to less than 9 hours: 6.3%
  • 9 hours or more: 2.0%

Under the targets used in Japan's Health Japan 21 (Third Term), “sufficient sleep duration” is defined as 6–9 hours for people aged 20–59 and 6–8 hours for those aged 60 and over.

In the 2024 survey, 56.0% of adults aged 20 and over fell within these ranges.

About eight in ten people reported getting adequate rest from sleep

In the same survey:

  • “Getting sufficient rest”: 24.8%
  • “Getting a fair amount of rest”: 54.8%

Together, 79.6% reported that they were getting sufficient or fairly sufficient rest from sleep.

These figures illustrate why it is useful to consider sleep duration and the feeling of being rested as separate but complementary measures.

What is sleep quality? Looking at the whole night's sleep

“Sleep quality” is a broad concept and can be assessed from several perspectives.

  • whether sufficient sleep duration is being obtained
  • how long it takes to fall asleep
  • how often or how long you wake during the night
  • how much time you actually spend asleep
  • sleep efficiency, or how much of your time in bed is spent sleeping
  • whether you feel rested in the morning
  • whether you experience strong daytime sleepiness

Japan's sleep guidance treats sleep duration as a measure of “quantity” and the feeling of being rested by sleep as an important indicator reflecting sleep “quality”.

Look at the whole night's sleep rather than deep sleep alone

A normal night includes lighter non-REM sleep, deep non-REM sleep and REM sleep, with these stages naturally changing throughout the night.

Because each stage has different physiological characteristics, it is useful to consider the overall night's sleep together with how you feel the following day.

What is the circadian clock?

The body has an approximately 24-hour circadian rhythm.

Many physiological processes change according to time of day, including:

  • sleep and wakefulness
  • body temperature
  • hormone secretion
  • metabolism
  • alertness

In the brain, the suprachiasmatic nucleus (SCN) in the hypothalamus acts as a major central circadian clock.

This clock adjusts its rhythm using timing signals from the environment.

One of the most important of these signals is light.

What is melatonin and how is it related to sleep?

Melatonin is a hormone produced mainly by the pineal gland in the brain.

Its secretion rhythm is regulated by the light-dark cycle and the circadian clock.

Melatonin levels generally rise at night and help signal that biological night has begun.

Rather than thinking of melatonin simply as the body's “sleeping pill”, it is more accurate to understand it as a hormone involved in circadian timing and signalling biological night.

Strong light exposure during the evening and night can influence the timing and amount of melatonin secretion.

Did you know? Melatonin sold as a supplement overseas is treated differently in Japan

In some countries, melatonin is sold as a dietary supplement.

In Japan, however, melatonin is classified as a substance used exclusively as a pharmaceutical ingredient.

When considering overseas products, it is therefore important to check how the ingredient is legally classified in the country where the product will be used or imported.

Using morning and evening light to support sleep rhythms

Light is an important timing signal for the sleep-wake cycle.

Bring in bright light in the morning and during the day

Exposure to bright light after waking helps signal morning to the circadian clock and contributes to regulation of the sleep-wake rhythm.

Adequate light exposure during the day is also related to night-time melatonin rhythms and sleep timing.

Opening the curtains after waking and, when possible, spending some time outdoors during the day are simple ways to strengthen daytime light exposure.

Reduce brightness toward bedtime

Strong light late at night can shift the circadian clock later.

Lowering room lighting before bed and creating a clearer contrast between daytime and night-time brightness can help provide the body with a stronger “night-time” signal.

Why can smartphones before bed affect sleep?

The relationship between smartphones, tablets and sleep is often discussed in terms of blue light.

Bright evening light containing shorter wavelengths can influence the circadian clock and melatonin secretion.

With smartphones, however, screen light, usage time and stimulating content can all matter.

Social media, videos, gaming and work emails can contribute to:

  • delaying bedtime
  • maintaining mental alertness
  • interrupting sleep through notifications
  • prolonged close-range screen exposure

Japan's sleep guidance recommends keeping smartphones and tablets out of the bedroom where practical and sleeping in a reasonably dark environment.

A practical approach is to manage both the brightness of the environment and the amount of digital-device use before bedtime.

Exercise and sleep

Daytime physical activity and exercise are among the lifestyle habits associated with healthy sleep.

Recent systematic reviews and meta-analyses have reported associations between exercise interventions and measures such as subjective sleep quality and sleep efficiency.

Many forms of exercise have been studied, including:

  • walking and other aerobic exercise
  • resistance training
  • yoga
  • Pilates

From a practical perspective, regular physical activity that you can maintain consistently is a useful place to start.

For evening exercise, adjust intensity and finishing time

Exercise in the evening can be incorporated successfully by considering intensity and timing.

Very vigorous exercise immediately before bed can leave body temperature and alertness elevated for some time.

Japanese sleep guidance suggests that when performing higher-intensity exercise, it may be helpful to finish around two to four hours before bedtime where practical and allow time to unwind afterwards.

Bathing and core body temperature: when is the best time for a bath?

Sleep and body temperature are closely connected.

During the evening, the body releases heat through the hands, feet and other areas as core body temperature begins to fall.

This change in body temperature is closely linked with sleep timing.

A warm bath temporarily raises body temperature, after which heat is released through the skin and core temperature gradually falls.

Japan's sleep guidance includes bathing around one to two hours before bedtime as one practical approach.

A meta-analysis of passive body heating also evaluated studies using warm baths or showers at around 40–42.5°C one to two hours before bedtime, including outcomes such as sleep onset and sleep efficiency.

A practical way to use this information is to choose a relaxing bathing routine and allow the natural temperature decline afterwards to take place before bed.

Meal timing and sleep

The sleep-wake rhythm is influenced not only by light, but also by the timing of everyday activities.

Meal timing is one of these factors.

A habitual large meal very late at night can also shift other parts of the routine later, including bedtime and breakfast time.

For people who regularly arrive home late, Japanese sleep guidance describes a split-meal approach, such as eating part of the evening meal earlier and having a lighter meal after returning home.

Think about the overall diet when considering sleep

Dietary patterns containing foods such as fish, vegetables, fruit, nuts and legumes have also been studied in relation to sleep.

For everyday nutrition, the practical foundation remains a varied diet that combines staple foods, protein-rich dishes, vegetables and other nutrient-dense foods.

How late is too late for caffeine?

Caffeine, found in coffee, black tea, green tea, energy drinks and other products, has stimulant effects.

Depending on the amount and timing, caffeine can influence:

  • how long it takes to fall asleep
  • deep sleep
  • night-time awakenings
  • total sleep duration

Caffeine has a half-life of around 3–7 hours

Japan's Good Sleep Guide states that the blood half-life of caffeine in Japanese people is approximately three to seven hours.

This means that some caffeine consumed in the late afternoon may still be present in the body at bedtime.

The speed at which caffeine is metabolised also varies between individuals.

Use 400 mg per day and evening timing as practical reference points

Japanese guidance recommends that adults keep total daily caffeine intake to no more than around 400 mg and, when focusing on sleep, keep intake lower in the evening.

People who are especially sensitive to caffeine may prefer switching earlier in the day to decaffeinated coffee or naturally caffeine-free drinks such as barley tea, black soybean tea or herbal tea.

Alcohol and sleep

Alcohol can make some people feel sleepy.

It can also temporarily shorten the time it takes to fall asleep.

As alcohol is metabolised later in the night, however, changes may include:

  • more frequent awakenings
  • lighter sleep
  • reduced sleep continuity

When considering alcohol as part of an overall sleep routine, it is useful to pay attention to both the amount and timing and observe how it affects the whole night's sleep.

How to use naps well

Naps can be useful in some situations for reducing daytime sleepiness and helping maintain alertness.

Appropriate napping has also been studied in shift work and other situations where normal sleep schedules are disrupted.

Long daytime sleep, however, can reduce sleep pressure later in the evening and may affect sleep onset or sleep continuity at night.

A practical approach is to adjust naps according to your schedule while keeping night-time sleep in mind.

For older adults in particular, maintaining daytime light exposure, physical activity and social activity can help create clearer separation between daytime and night-time routines.

How should we think about weekend sleep?

Sleeping longer on weekends after shorter weekday sleep is often called weekend catch-up sleep.

A little extra sleep on days off may help reduce short-term sleepiness or tiredness.

For chronic sleep restriction, research looks at both the potential recovery from weekend sleep and the importance of obtaining adequate sleep during the working week itself.

Weekend sleep involves both recovery and maintaining rhythm

Sleeping longer on days off can be one way of partially compensating for shorter sleep during the week.

At the same time, if wake time shifts much later on weekends, the timing of sleep can become noticeably different between workdays and days off.

This difference is known as social jetlag.

A practical way to use weekend sleep is to secure enough sleep during the week while keeping weekday and weekend wake times reasonably close and maintaining a consistent daily rhythm.

Light, temperature and noise in the bedroom

Sleep is influenced not only by internal biology but also by the environment in which we sleep.

Light

A reasonably dark environment is generally helpful during sleep.

Consider room lighting as well as smartphones, televisions and strong outdoor light entering the bedroom.

Temperature

Both excessive heat and excessive cold can influence sleep onset and continuity.

Season, bedding and sleepwear can all be adjusted to create a comfortable thermal environment.

Noise

The brain does not completely stop responding to sound during sleep.

Traffic noise, television sound and notification alerts can all disturb sleep in some people.

Rather than focusing on one “perfect” bedroom temperature, it is more useful to create a comfortable environment by considering light, temperature, sound and bedding together.

How should you read a smartwatch sleep score?

Smartwatches and smart rings can now estimate information such as:

  • sleep duration
  • sleep stages
  • night-time awakenings
  • heart rate
  • sleep scores

These devices can be useful for observing patterns in bedtime, wake time and sleep habits.

Clinical sleep testing may measure multiple physiological signals including brain waves, eye movements, muscle activity and breathing.

Consumer wearables usually estimate sleep from signals such as movement and heart rate.

For this reason, rather than focusing heavily on a single result such as:

“I had 12 minutes less deep sleep last night,”

it can be more useful to look at:

  • bedtime and wake-time patterns
  • total sleep duration
  • trends over several weeks
  • how rested you actually feel

Use sleep scores as information for reviewing everyday habits

Sleep scores and sleep-stage estimates can provide a useful prompt to review daily habits.

Combining device data with your own sense of restfulness, daytime function and overall wellbeing can make sleep tracking more practical.

How should we think about sleep supplements?

Products marketed around sleep may contain ingredients such as:

  • magnesium
  • GABA
  • glycine
  • L-theanine
  • botanical ingredients

Some of these ingredients have been studied in human research in relation to sleep or relaxation.

Studies, however, evaluate specific ingredients, doses, populations, durations and outcome measures.

When comparing supplements, useful questions include:

  • which ingredients are included
  • how much is provided per daily serving
  • which specific source or raw material is used
  • whether ingredients overlap with other supplements you take
  • what claims are permitted for that particular product

Sleep supplements and sleep medicines have different purposes and regulatory positions

Foods and dietary supplements differ from medicines used to treat conditions such as insomnia in their purpose, evaluation, dosage and directions for use.

In Japan, melatonin is treated as a pharmaceutical ingredient, even though it may be sold as a supplement in some overseas markets.

When considering an overseas product, check not only where it is sold but also how the ingredient is legally classified where the product will be used or imported.

For more about magnesium, see: What Is Magnesium? Functions, Magnesium-Rich Foods, Daily Intake, Deficiency & Supplements.

Japan's Foods with Function Claims: check the notification details

In Japan, some foods are marketed with sleep-related functions under the Foods with Function Claims system.

Under this system, businesses submit information to the Consumer Affairs Agency regarding safety and scientific evidence supporting the notified function before marketing the product with that function claim.

When comparing these products, useful points to check include:

  • the notification number
  • the functional ingredient
  • the recommended daily intake
  • the exact wording of the notified function claim

Even when two products contain ingredients with the same name, such as GABA or L-theanine, the ingredient source, amount, scientific evidence and notified function may differ from product to product.

So rather than looking only for wording such as “contains GABA” or “contains theanine”, it is useful to check what has actually been notified for that specific product.

Japan's Consumer Affairs Agency database allows consumers to review notification information and supporting documents for individual Foods with Function Claims.

The Foods with Function Claims system is different from Foods for Specified Health Uses (FOSHU) and does not involve government approval of each product's function through individual pre-market assessment.

A practical approach is to read the notified wording carefully and consider the product as part of an overall dietary and lifestyle routine.

When ongoing sleep problems deserve closer attention

Some people may continue to feel poorly rested even after improving sleep duration and everyday habits.

Possible underlying factors can include:

  • insomnia
  • obstructive sleep apnoea
  • restless legs syndrome
  • circadian rhythm sleep-wake disorders
  • other physical or mental health conditions

Medical assessment can be especially useful if you experience:

  • persistent loud snoring
  • observed pauses in breathing during sleep
  • strong daytime sleepiness despite adequate time in bed
  • sleepiness while driving
  • ongoing difficulty falling asleep or staying asleep that interferes with daily life
  • uncomfortable leg sensations that make sleep difficult

If sleep quality is a concern, reviewing sleep duration, daily rhythm, sleep environment and overall health step by step can help clarify what type of support may be appropriate.

Summary: look at both sleep duration and how rested you feel

Sleep is a fundamental form of rest that supports both brain and body function.

The key points are:

  • Sleep is associated not only with rest but also with memory, metabolism, endocrine function, immune function and other biological processes.
  • Sleepiness is regulated by both sleep pressure, which increases with time awake, and the approximately 24-hour circadian clock.
  • Non-REM and REM sleep alternate cyclically through the night.
  • Sleep cycles are often around 90–120 minutes, but there is substantial individual and within-night variation.
  • For adults, around 6–8 hours is a useful central range while identifying the amount of sleep that suits the individual.
  • Sleep quality is best considered using sleep duration, how rested you feel and your daytime functioning together.
  • Morning and daytime light, exercise, bathing, caffeine, meal timing and evening light all contribute to the overall sleep-wake rhythm.
  • When comparing sleep-related products, check the ingredients, amounts, product category and any notified function claims.

A practical approach to sleep is to secure enough sleep for your own needs, notice whether you feel rested in the morning, and check whether you can function comfortably during the day.

From there, gradually adjusting morning light, daytime activity, evening brightness, bathing, meals, caffeine and the bedroom environment can help you build a sleep routine that fits your lifestyle.

Related Articles

Main References

  • Ministry of Health, Labour and Welfare, Japan. Good Sleep Guide for Health Promotion 2023.
  • Ministry of Health, Labour and Welfare, Japan. National Health and Nutrition Survey 2024.
  • Ministry of Health, Labour and Welfare, Japan. Health Japan 21, Third Term.
  • Ministry of Health, Labour and Welfare, Japan. Standards concerning the scope of pharmaceutical ingredients and food-drug classification.
  • Consumer Affairs Agency, Japan. Foods with Function Claims.
  • Consumer Affairs Agency, Japan. Foods with Function Claims notification database.
  • Scammell TE, Arrigoni E, Lipton JO. Neural Circuitry of Wakefulness and Sleep. Neuron. 2017.
  • Bastianini S, et al. Ultradian sleep cycles: Frequency, duration, and associations with individual and environmental factors. Sleep Health. 2023.
  • Zhou X, et al. Effects of exercise on sleep quality in general population: Meta-analysis and systematic review. Sleep Medicine. 2025.
  • Xie W, et al. The optimal exercise intervention for sleep quality in adults: A systematic review and network meta-analysis. Preventive Medicine. 2024.
  • Haghayegh S, et al. Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis. Sleep Medicine Reviews. 2019.
  • Weaver MD, et al. The importance of sleep regularity: a consensus statement of the National Sleep Foundation sleep timing and variability panel. Sleep Health. 2023.
  • Hsiao FC, et al. The sleep paradox: The effect of weekend catch-up sleep on homeostasis and circadian misalignment. Neuroscience & Biobehavioral Reviews. 2025.
  • Zhou Y, Xue F. Can weekend catch-up sleep repay the sleep debt? Sleep and Breathing. 2025.

Important Information

This article is intended to provide general educational information about sleep, lifestyle and nutrition.

It is not intended to diagnose, treat, cure or prevent disease, and it does not claim that any particular food or supplement treats insomnia, sleep disorders or other medical conditions.

Research discussed in this article was conducted under specific conditions involving particular populations, ingredients, doses, study periods and assessment methods. The findings should not be interpreted as guaranteeing the same results for any individual commercial product.

For Foods with Function Claims, check the functional ingredient, recommended daily intake, notified wording and relevant precautions for the individual product.

If you experience persistent loud snoring, pauses in breathing during sleep, strong daytime sleepiness, ongoing insomnia or other sleep problems that interfere with daily life, consult a doctor or other qualified healthcare professional.

If you are receiving medical treatment, take medication, or are pregnant or breastfeeding, consult a doctor, pharmacist or other qualified healthcare professional before using sleep-related supplements or health products.

Category
Beauty & Wellness Guide

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