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Biological Rhythm

  Biological rhythm is the term used in the life sciences to describe the relationship between fluctuating biological phenomena and periodic changes in the environment, such as the regular passage of night and day and seasonal changes. Periodic phenomena, such as breeding seasons and menstrual cycles, and daily patterns, such as flowers opening in the morning and closing at night, are examples of biological rhythm. Cycles with a period of 24 hours are called circadian rhythms; they are influenced by the daily cycle of light and dark, but will continue if the organism is removed to a laboratory situation where light is continuous and temperature constant and will be adjusted, after a brief delay, if it is moved to a new time zone. Some periodic events are not clearly linked to environmental cycles: the 17-year cycle of the reproduction of the cicada, for example, always occurs at the same time of year but there is no clear environmental stimulus for the 16-year wait. RB

See also chronobiology; phenology.



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