[Part5]BIOLOGICAL CLOCK: Researchers study human responses to 'social jet lag'

photo:Hiroyuki Kodera

The human body has an internal clock that adjusts biological rhythms on a roughly 24-hour cycle to repeat processes such as sleep and activity. Our brains cooperate with “clock genes” in our cells to regulate body temperature, hormone secretions and more.

For example, our blood pressure begins to rise with the sun, in preparation for the activities we perform after we wake up. Our sympathetic nervous system, which regulates our "fight-or-flight response," is most active between 3 p.m. and the evening. And in the one to two hours before we go to sleep, a hormone called melatonin that causes drowsiness is secreted to set us on the path to slumber.

The rhythms of our internal clocks differ subtly from person to person. There are “night people” whose cycles last longer than 24 hours, and “morning people” whose cycles are shorter.

In the past, it was only possible to study such differences in our individual biological clocks through long-term observation of subjects isolated in rooms where they are unable to distinguish the time. However, a range of methods have been developed recently that make such research much easier.

“The aim is to diagnose social jet lag,” says Kazuo Mishima of the National Center of Neurology and Psychiatry.

“Social jet lag” is a condition in which our biological clocks fall out of sync with our living hours as dictated by our actions in society, resulting in physical ailments and sleep and waking disorders.

A health ministry study found that nearly 30 percent of all workers work on some kind of shift basis. There is also a tendency to go to sleep at a later hour. Actions such as going to the convenience store or staring at a computer screen in the middle of the night gradually throw our biological clocks out of alignment.

Normally, such deviations in our biological clocks are reset when we bathe in the strong light of morning.

“However, when you work a night shift over a long period of time, your biological clock becomes more and more out of step, which has a negative impact on your health,” Mishima says.

There are treatments to fix our biological clocks, such as bathing in strong light from a lamp or receiving injections of sleep hormones. Even so, until recently it has been difficult to diagnose the degree of and tendency toward misalignment of our biological clocks, so treatment was unavoidably hit-and-miss. If it becomes possible to detect the individual differences in our biological clocks, we will be able to receive treatment tailored specifically to our needs.

Mishima's team has developed a method of diagnosing individual differences based on a skin sample measuring 2 x 5 millimeters. The clock genes in the skin cells are illuminated using a special technique, and the rhythm with which the light is emitted is used to investigate misalignments in the subject's biological clock.

Elsewhere, Hiroki Ueda and his colleagues at Riken have created a method for measuring misalignments based on blood samples, and Yamaguchi University professor Makoto Akashi is developing a technique that uses hair root cells.

Diagnostic procedures such as these have been made possible by accelerated research into clock genes.

The human clock gene was first pinpointed in 1997. In the 16 years since, around 20 genes have been discovered that are associated with the biological clock, and we have developed a greater understanding of its elaborate mechanisms.

It is said that the clock genes in cells throughout our bodies beat out rhythms autonomously as “slave clocks,” while the “master clock” in the supraoptic nucleus behind our eyes controls the slave clocks and commands the entire system. The jet lag we experience when traveling overseas is thought to develop because it takes longer for the slave clocks to adjust to a new time zone than the master clock.

“Cellular-level analysis has made it increasingly clear that high blood pressure and many other lifestyle-related diseases are induced by a misaligned biological clock,” says Kyoto University professor Hitoshi Okamura, a member of the research team that was the first to determine a type of clock gene. “Applied research for preventing and treating such illnesses is likely to broaden in scope from now on.”

photo:Hiroyuki Kodera


Apart from biological clocks, animals have other rhythms such as heartbeats and breathing. The smaller the creature, the faster the rhythm, and larger beasts tend to be slower.

For instance, the heart of a house mouse beats more than 600 times per minute, compared to 200 times for house cats and 60 to 70 times for humans. A three-ton elephant only manages 20 heartbeats, each emitting a powerful pulse once every three seconds.

“If a body weight is 10 times larger, its pulse is 1.8 times slower,” says Tokyo Institute of Technology professor Tatsuo Motokawa, author of the book “Zo no Jikan, Nezumi no Jikan” (Elephant Time, Mouse Time). “This rhythm is said to be proportional to the one-quarter power of body weight.”

Biological rhythms become slower as body size increases because energy consumption per cell is lower. Smaller creatures are more vulnerable to changes in their environment, so they require an exceedingly large amount of energy to maintain their body temperature, and their lifespan is consequently shorter. “Every living thing has a biological clock that operates on a 24-hour cycle, but in terms of energy metabolism, you could say that the smaller the animal, the more quickly that time passes for it.”

Degrees of energy metabolism are also related to human beings' perception of time.

Take the saying that time passes by faster as you get older. There are various theories as to why this is. Some people believe it is because the ratio of one year in your lifespan decreases as you increase in age, while others attribute it to a gradual decline in stimulation as you get older because you experience fewer things for the first time. There is also the view that it is due to being unable to perform the same amount of tasks in a set period of time as our energy metabolisms deteriorate with age.

When we are young, we can engage in various activities within a short time. It seems that time is flying by while we are in the middle of doing them, but when we look back on that time, we remember it as being richer in content and longer. On the other hand, when we grow older, everything we do takes longer than it used to, and when we look back on that time, we remember it as being meager in content and shorter.

What about the flow of time in our society?

“In modern times, everything has increased in pace due to mass energy consumption,” Motokawa says. “The human mind and body is having increasing difficulty keeping up with that speed.”

The bodies of living beings have mechanisms that suppress our consumption of energy in order to slow the passage of time. Some animals hibernate, and some plants can stay dormant for years as seeds. Perhaps it is time that humans also discovered the value of slowing down.

(This article was written by Ikuya Tanaka, senior staff writer of The Asahi Shimbun.)


At a park not far from Tokyo's JR Kinshicho Station, there is a belfry-shaped artwork about the size of a public telephone box. It is a monument that indicates there was once a “jishou” nearby in the Edo Period (1603-1867), when the area was known as “Honjo.”

Jishou were bells used to announce the time for the benefit of the denizens of Edo (old Tokyo) who did not own timepieces, and the Honjo jishou had a particularly strong connection to a historic event that is known to most Japanese: The raid on the Kira residence by masterless samurai of the Ako Domain (present-day Hyogo Prefecture) in January 1703.

It was an era when you couldn't check your watch to make sure you were on time to meet someone.

“Kuranosuke Oishi and his comrades probably relied on this jishou to synchronize their meetings at three different hideouts and their raid on the residence of (shogunate official) Kozukenosuke Kira,” says Yasuo Nakajima, chairman of history research group Chuogishikai (Central Loyal Retainer Association).

During this period, a time system was in use called “futeijiho” (unfixed time method). Day and night were each divided into six units called “ittoki.” As the length of daytime differs depending on the season, the length of an ittoki was also variable. The role of the jishou was to announce the beginning of each ittoki.

The shogunate's jishou operators are said to have rung their bells in time with such devices as a “wadokei” (Japanese clock), essentially a Western-style mechanical desk clock customized to work on the futeijiho system, or a “koban-dokei” (incense-burning clock), which displayed the passage of time according to the speed with which incense burned.

Apart from the Honjo jishou, there were several other jishou located in the city of Edo. Local residents who benefited from them were asked to pay a monthly fee to keep them running. Areas without jishou relied on temple bells to fulfill the same function.

People ate, left for work, and headed for home when the jishou pealed. As they were used by whole communities, the daily schedules of local residents became synchronized.

After the Meiji Restoration, Japan adopted the solar calendar and time system used in the West. Production of wall clocks also got under way, but it took a little while before they became fixtures in ordinary households. In response to this situation, the Grand Council of State issued a decree in 1871--the fourth year of the Meiji Era (1868-1912)--that a single cannon round was to be fired daily at noon, and loud bangs were subsequently heard around the country. The decree remained in place until 1929.

In Tokyo, a cannon was installed at the Imperial Palace, and the single boom of a blank shell came to signal the end of work for the morning.

(The last portion of the article was written by Shinya Wake, GLOBE staff writer.)

Translated by The Asahi Shimbun AJW. More related stories available at

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