Why Don't Circadian Rhythms Coincide With the 24-Hour Day?

Found almost ubiquitously in living organisms, circadian rhythms are generated by body clocks with natural periods ranging roughly from 22 to 28 hours. However, they rarely match the 24-hour length of a day (hence the word "circadian," meaning "about a day"). This is strange, since species whose body clocks are resonant with the Earth's 24-hour cycle of light intensity and temperature variations ought to be the best ones for adapting to the environment and organizing daily activities.

To address this issue, a physicist in Japan (Hiroaki Daido, Kyushu Institute of Technology, now at the University of Osaka Prefecture, daido@ms.osakafu-u.ac.jp, 011-81-722-54-9366) has devised a mathematical model that explores competition between species with body clocks of different periods. Daido makes two major assumptions in his model: First, the population growth rate of a species depends on the time difference between its body clock period and the 24-hour day (for example, a creature vulnerable to harmful ultraviolet rays during the day will have a maximum growth rate if it has a 24-hour cycle and therefore stays perfectly nocturnal). Second, the amount of competition between pairs of species becomes more severe with a smaller time difference between their body clocks (two species looking for food will have an easier time if they do it 12 hours apart).

Daido's model shows in particular that a 24-hour body clock can actually turn out to be a disadvantage as long as the benefits of being in sync with the environment are not large enough. That's because competition with other species turns out to be most intense for species with 24-hour body clocks. Daido's model can also address other biological rhythms such as circannual rhythms which control, for example, animals' hibernation timings.

However, he points out that other factors, such as the effects of natural disasters, may also have contributed to the existing circadian rhythms, and he calls for testing the results of his model with biological observations and experiments. (Daido, Physical Review Letters, 23 July 2001; text at Physics News Select).