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 rhythm s, 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 rhythm s, 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 rhythm s, 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.

External link

• The University of Virginia offers an online tutorial http://www.cbt.virginia.edu/tutorial/TUTORIALMAIN.html on chronobiology.