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Chronobiological Control of Light Environment: The Interface of the Internal Microcosm and the External Natural Environment

Emi Koyama (D. Eng.,

Senior Researcher, Home Appliances R&D Laboratory, Matsushita Electric Works, Ltd.

現職: Associate Professor, Department of Design Engineering & Management, Kyoto Institute of Technology )


The function of the biological clock is to help creatures adapt to the environmental changes which occur along with time changes such as day periodicity and year periodicity. The approximately 24-hour rhythm, calling "circadian rhythm", is one of the most well known types of biological rhythm. In the circadian cycle, we alternatively sleep and wake, controlling our body temperature and hormone secretion. Circadian rhythm disorders can confuse vital functions such as sleep, exercise and diet; it may also cause life-style disorders such as fatigue, stress, obesity, and autonomic imbalance.

It is known that the light environment greatly modulates circadian rhythms. We believe that an effective method of adjusting the circadian rhythm, which is deeply related to our health, is to control the light environment.

Light Modulates Our Biological Clock

There are two central nervous systems through which light from the retina affects the body: that of the biological clock, and that which controls sleep-wake system. When we see light, the secretion of a sleep-inducing hormone is inhibited and the sympathetic nervous system becomes active. These changes enhance alertness. Thus, light is strongly related to sleep-wake system. The influence of light depends not only on its instantaneous illuminance but also on the dose of light exposure (= illuminance x hour x lighting efficiency).

The time of day also influences the effects of light on the biological clock. When light enters the eyes shortly after the nadir of the body temperature (generally just before dawn), the biological clock is advanced. On the other hand, the clock is delayed when we see light late at night. Generally the human circadian rhythm has a period of slightly longer than 24 hours and it is said that we modulate the phase of the rhythm according to the light we see.

To Improve Quality of Awakening

The illuminance that we usually experience ranges from 0 to 100,000 lux, which is the illuminance at the outside on a day with fine weather. In contrast, the illuminance in a very bright office is only about 1,000 lux. Thus, from a biological point of view, daytime light is too dark and nighttime light is too bright for people in an urban indoor environment. In a word, there is no significant change throughout a whole day in the light environment of modern people.

I propose to control the light environment according to the time of day in order to stabilize circadian rhythm on the basis of physiological awakening function of light. Even if it is impossible to simulate a natural light environmental change for a full day, it may be sufficient to control light in several classified phases such as before waking up, during the daytime, in the evening and during sleep.

Natural dawn light gets brighter gradually over period of 2 or 3 hours, allowing people to make a smooth transition from sleep to wake. However, because there are places without morning sunlight, and because the natural time of sunrise may not precede our wake-up time, control of the artificial light environment is necessary to stabilize our circadian rhythms and to improve our lifestyle.

We conducted an experiment in which the illuminance at the bedside was increased gradually 30 minutes before wake-up time for a period of two weeks. Our results show stabilization in core body temperature rhythm, which had been rather unstable when the subjects woke up in a dark room in the baseline period of one week; additionally, subjective evaluation of sleep improved to "waking up feeling good" and "sound sleep." Furthermore, another experiment with polysomnographic recordings indicated that gradually increasing morning light changes the sleep state just before wake-up time lighter.

Nevertheless, for optimum interaction between the external environment and the internal body system, it is necessary to control the light environment according to each individual's body conditions by measuring the circadian rhythms and the dose of light exposure. We need to develop an effective measurement system and create a light environment adjusted chronobiologically.

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