A biological cycle, or rhythm, that is approximately 24 hours long is called a(n) ______ cycle.

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A circadian cycle is a biological rhythm that repeats itself approximately every 24 hours. In contrast, an infradian cycle lasts longer than 24 hours (e.g. menstrual cycle), a diurnal cycle is specific to animals that are active during the day (e.g. birds), an ultradian cycle is shorter than 24 hours (e.g. REM sleep cycles), and a supradian cycle is longer than a day but shorter than a year (e.g. seasonal breeding patterns in animals).

A Biological Cycle, or Rhythm, That Is Approximately 24 Hours