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Light is One of the Keys to a Good Night Sleep

Light is one of the keys to a good nocturnal sleep.

A sustained circadian sleep-wake rhythm prevents us from sleep disorders.

According to the statistics of a research institute in 1991,"One out of three in America suffers from insomnia." You might well understand the situation, since the American specialists have pointed out that the Japanese people are more in need of sleep than the Americans are. Surely, Japanese in the present age live with shorter sleeping hours than before. This is not simply because there is less time for sleep. Rather, people do not keep regularity in sleep-wake timing together with a different lifestyle from before. It is usual for people to stay late at the office or at school, and they dine and play late at night. Even if they stay at home, most are absorbed in watching TV or videos, or playing video games. In addition, countries around in the world are always connected in real time through the Internet. As a result, there is no longer a boundary between day and night. We have just 24 hours a day. The longer you stay awake, the shorter you sleep, of course. What kinds of effects are made on our life by a chronic lack of sleep and what can we do to solve this problem?

The role of sleep

When you consider the effects of not getting enough sleep, you will understand what good sleep is for. You will be in a bad condition mentally and physically, feel continually fatigued and irritated, and suffer from impaired judgment and concentration. Imagine trying to drive in such a state. It is known that sleep rests both the mind and body, and it maintains the function of the brain and the immune system. Human life would never be organized without proper functioning of the brain. In short, sleep is an element that keeps the brain working normally.

However, I regret to say that we don't know fully the details of sleep. Not only do we lack complete knowledge of what sleep is, but we also may get incorrect information about how to sleep well and about the difficulties in sleeping well. Don't you misunderstand that you are suffering form insomnia because you can't sleep well, that you feel sleepy in daytime because you lack in nighttime sleep, and that you have difficulty in getting up in the morning because your blood pressure is low? Let's think about good sleep once again.

Circadian rhythm of sleep

We hear the term "biological clock." Most organisms have this function, which creates the cycle of day and night in the body and makes the body adapted to the 24-hour environmental cycle. Each creature makes a rhythm dividing one day into two terms, one for activity and the other for rest. This is called a circadian rhythm. It is known that the free-running circadian rhythm period of human beings is a bit longer than 24 hours. The reason why is not clear yet, but it nonetheless seems to be very important for human life. Even though people feel sleepy when they get up, they become awake while they have breakfast and wash their faces in sunlight. A one-hour time difference is rectified unconsciously.

One of the important elements of circadian rhythm is a sleep-wake cycle. In addition to forming part of the daily cycle, sleep has another function: to make up for sleep insufficiency. If we have shortened sleep in a previous night or we are quite busy working and tired in the daytime, we will then have a particularly deep sleep when the bedtime comes. Much to my interest, when we nap too much or idle our time away in daytime, the following nighttime sleep is not so good and it's harder to wake up in the next morning. As described above, our physical and mental health conditions are adjusted by sleep. Let's take a look at the physiological structure of sleep.

REM Sleep and NREM (Non-REM) Sleep

REM stands for rapid eye movement. The eyes move and dreams occur during REM sleep. The other part of sleep is called NREM sleep, in which the brain rests. Human sleep is composed of cycles alternating between these two forms of sleep.

It is a mistake to simply regard REM sleep as light sleep and NREM sleep as deep; each of these forms has its own significance. Normally, NREM sleep starts when we fall asleep. NREM sleep calms the brain and is divided into four stages, from drowsy to deep sleep. The first period of NREM sleep is the deepest sleep, which is followed by the first period of REM sleep. REM sleep is for refreshing and activating the brain, and this is why the eyes move. Though the body seems to rest in REM sleep period, heart rate, blood pressure and breathing rate are fluctuating. One sleep cycle, which is defined as the beginning of a NREM sleep to the end of a following REM sleep, takes about 90 minutes. The most suitable point at which to wake up and begin daily activities is at the end of a REM sleep after a proper length of sleep. This suggests that it is best to wake up after 6 to 8 hours of sleep. The deepest NREM sleep is normally observed during the first three hours of sleep. This is an interesting point of the sleep mechanism; the deepest NREM sleep comes first in order to recover from fatigue, and the following sleeps are controlled to refresh the brain. Napoleon is said to have slept only three hours a day. This may be a good example of using this first sleep effectively.

Normal Sleep and Sleep Disorders

It is difficult to define a good or normal sleep, since each person has a different sleep quality. One thing for certain is that a comfortable awakening means that you have slept adequately in the previous night. You do not have to care so much about the length of your sleep or the time at which you go to sleep. The problem is that our present lifestyle makes it difficult to obtain such a comfortable sleep.

In recent times we have come to hear the term "sleep disorders" which, in brief, means that a person cannot sleep normally. The symptoms of sleep disorders are various. A diagnostic classification is showed in the table. Insomnia is the best-known disorders, that unables to get a sufficient quantity or quality of sleep. Moreover, the recent serious topic is the circadian rhythm sleep disorders, particularly Delayed Sleep Phase Syndrome (DSPS). Those who suffer from DSPS cannot sleep smoothly at night and cannot get up in the morning, so the activities of the following day are hindered. The worst case is when they not only are unable to go to work or school, but also cannot communicate with others.

An irregular lifestyle may disturb the biological clock and may finally cause the disruption of the circadian sleep rhythm. If a person wakes up and uses the brain during the natural nocturnal sleep time, the brain does not act adequately during the daytime, which is the natural active time. There is also a hypothesis that these disorders are effected by an inability of the body to make the growth hormone needed for the body's construction and maintenance, which is secreted during the period of early night NREM sleep.

Morning Light Is Important

Previous research has indicated that the method of light exposure that is preferable for human awakening is not a sudden, bright illumination but rather a light that begins dimly and increases illuminance gradually. Hence, dawn is an ideal time to wake up. However, given today's lifestyles, people do not often awaken under these conditions. Not only curtains and the position of the room but also a late bedtime make it difficult to wake up with the sunrise. Still now a sleep environment in which you are exposed to gradually increasing light is desirable if possible. As discussed above, it is preferable for the brain activity to maintain the day and night regularity of the circadian rhythm, and to wake up at the end of a REM period after adequate NREM-REM cycles. The brain would work healthily with this proper regularity. The same thing is said for the time just before sleep. In the evening, you have to try to create a situation that makes it easy for the brain to relax before sleep. Watching TV, video games, reading or listening to stimulating things is not advisable. You can control such things even in the present lifestyle.

In the present lifestyle of digitalization, we regard it as natural that everything operates according to "ON or OFF" mode, but the human body is not so digitized as machines. There is no certain time when the morning starts; the sun gradually sets and rises as it has for eons. Since the natural circadian cycle is programmed in the human body, our health requires us to mold a lifestyle that is suitable for this cycle.

Fig. Caption

Fig. 1 Time elapse of REM sleep and NREM sleep cycles. (Borbely, Alexander A. 1985. Nemuri no Nazo (Translated from "Das Geheimnis des Schlafs") Doubutsu-sha.)

時刻 Time

レム睡眠 REM sleep

ノンレム睡眠 NREM sleep

覚醒 Awakening

Classification of Sleep Disorders (睡眠障害のいろいろ)


*Intrinsic sleep disorders

1. Psychophysiological insomnia

2. Sleep state misperception

3. Idiopathic insomnia

4. Narcolepsy

*Extrinsic sleep disorders

1. Inadequate sleep hygiene

2. Environmental sleep disorders

3. Altitude insomnia

4. Adjustment sleep disorders

*Circadian rhythm sleep disorders

1. Time zone change (Jet lag) syndrome

2. Shift work sleep disorders

3. Irregular sleep-wake pattern

4. Delayed sleep phase syndrome


*Arousal disorders

*Sleep-wake transition disorders

*Parasomnias usually associated with REM sleep

*Other parasomnias

Sleep disorders associated with mental / psychiatric disorders

*Associated with mental disorders

*Associated with neurological disorders

*Associated with other medical disorders

Proposed sleep disorders


Fig. 2 Schematic presentation of the circadian rhythm and light  (生体リズムの図式

Light stimulates the brain through the eyes and affects awakening and circadian rhythms.



睡眠中も、まぶたを通して〜 The brain perceives light through the eyes during sleep.

覚醒作用がある、〜 Direct stimulation to the brain that has an awakening effect

覚醒を助ける変化 Changes to help awakening

*The body temperature rises.

*Melatonin secretion is suppressed.

睡眠と覚醒のサイクル Sleep-wake cycle

生体リズム Circadian rhythm (biological clock)

A phenomenon whose cycle is around a day

※ メラトニン:〜 *Melatonin is a hormone that has a hypnotic effect.

Fig. 3 Relation between the morning sunlight and awakening

There seems to be a clear correlation between awakening and morning sunlight through the window.

In a room that catches the morning sunlight, the awakening time becomes later as the sunrise becomes later.

日の出時刻 Sunrise time

覚醒時刻 Awakening time

朝日が入る場合 When the morning sunlight is caught.

遮光した場合 When the morning sunlight is shielded.

出典:〜 (Yanase, T. et al., 1988 Nemuri to Shinshitsu no Kagaku (The Science of Sleep and Bedroom). Matsushita Electric Works, Ltd. Technological Laboratory.)

Fig. 4 The awaking function of a bedside lamp "ASSA "

"ASSA", an awaking bedside lamp made by Matsushita Electric Works, Ltd., simulates morning sunlight, which becomes bright gradually. It lightens the bedside little by little for 30 minutes so that you can awake with the sleep rhythm.

お休み前 Before sleep

起床30分前 30 minutes before the hour of rising

起床時刻 Arise time

さわやかな目覚め Comfortable awakening

起床時刻に目覚ましセット Set the alarm for the wake-up time.

まず、うっすら点灯〜 A slight light appears, imitating the beginning of the dawn.

ゆっくり明るさアップ〜 The light becomes brighter little by little, just as with natural dawn, as the wake-up time approaches.

100%点灯&アラーム音〜 The light becomes fully bright and the alarm rings. The beep becomes gradually stronger. Even if you turn the "snooze" switch off, it will wake you up again after five minutes with the light stimulation and beep.

浅い眠りが〜 Light sleep becomes more frequent.

交感神経が〜 Sympathetic nerves become active.

体温が〜 Body temperature increases.

目覚めのための〜 The preparation for awakening is done.

起床 Getting up

スヌーズ Snooze

照度 Relative illuminance

起床時刻30分前 30 minutes before wake-up time.

タイマーセットした起床時刻 The wake-up time is set with the alarm.

時間(分) Time (min.)

快適な目覚めを助けて〜 A special bedside lamp to make "gradual light," which helps you to induce comfortable awakening and may prevent sleep disorders

「漸増光」を主照明で〜 A ceiling light by which "gradual light" is produced, which is already made practicable.

(Right) Remote-control device

Fig. 5 Difference between awakening in a dark room and a room with morning sunlight

暗い部屋での〜 Awakening by only sound stimulation in a dark room

朝の光が入る部屋〜 Awakening in a room with the morning sunlight

覚醒 Awakening


ぐっすり Deep sleep

眠りの深さ The depths of sleep

時間 (hours)

There are two types of sleep: REM sleep, when the brain works, and NREM sleep, when the brain rests. People sleep in a cycle that alternates between these two forms of sleep, and the sleep quality becomes gradually lighter as the awakening time, the morning, becomes closer. When the room catches the morning sunlight and becomes brighter little by little, the body does not return to a deep sleep, and it becomes easier to wake up comfortably. But when the room is kept dark, the brain easily goes back to a deep sleep. If the alarm beeps at that time, it will be difficult to wake up.


Cooperator in this report:

Matsushita Electric Works, Ltd.

Emi Koyama, Ph.D. (Engineering)

Senior researcher

Home appliances R&D laboratory

Naoya Yamamoto


Residential lighting division

International operations group

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