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Chronobiology is a field of biology that examines time-related phenomena in living organisms. These cycles are important in many essential biological processes that occur in a "scheduled" fashion, such as eating, sleeping, mating, hibernating, migration, and cellular regeneration.
The most important rhythm in chronobiology is the circadian rhythm, which refers to the 24-hour daily biological cycle; however, many other important cycles are also studied, including:
- Infradian rhythms, which are long-term cycles, such as the annual migration or reproduction cycles found in certain animals or the monthly menstrual cycle of human females.
- Ultradian rhythms, which are short cycles, such as the 90-minute REM cycle in sleep or the 3 hour cycle of growth hormone production. They have periods of less than 24 hours.
- Tidal rhythms, commonly observed in marine life, which follow the (roughly) 12-hour transition from high to low tide and back.
Related to, but not part of, chronobiology is the unsubstantiated theory of biorhythms, which are said to be a set of cyclic variations in human behaviour. The theory's basis lies in physiological and emotional cycles. Some people consider it pseudoscience and others protoscience.
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