Do you know that we have “clock” in our body? We have a clock called “biological clock”. There are a variety of clocks depending on the period. Among them, we especially study “circadian” clock that displays oscillations of about 24 hours. The 24-hour rhythm called circadian rhythm. The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day".

Rhythmicity in organisms

Circadian rhythms are the fundamental phenomena of life that are found in most living things, including microorganism, plants, animals and humans. The rhythms are produced by the rotation of the earth on its axis. Some organisms show circannual (a year) and circalunar (a month) rhythms produced by the period of revolution of earth and the period of revolution of moon, respectively.

The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were kept in constant darkness. This first experiment showed an endogenous clock existed in the plant. In the present day, it is considered that organisms inhabiting the earth have been getting various kinds of rhythmicity in order to adapt to environment over a long period of time.

Day-night variations in human physiological functions

　From a lot of knowledge, we have begun to understand the mechanism of biological clock. However, many questions still remain unclear. We are hardly working to find the answer of the question, “How can animals keep circadian rhythms accurately?” Also, as you can see in Fig.1, that shows many physiological functions have day-night variations. Now, chronotherapy is getting attention as a new medication that are administered according to a schedule that corresponds to a person's rhythms in order to maximize effectiveness and minimize side effects of the therapy. In our laboratory, we study to find out a time-dependent pharmacological effect of a medicine. Also, lipid metabolism has day-night variations. We are now working to develop more efficient technologies in stockbreeding and aqua farming, for example, changing of feeding time and finding more efficient light schedule in the field in producing domestic animals and cultured fishes.