Chronobiology

Chronobiology is a field of science that examines periodic (cyclic) phenomena in living organisms and their adaptation to solar and lunar related rhythms. These cycles are known as biological rhythms. "Chrono" pertains to time and "biology" pertains to the study, or science, of life. The related terms chronomics and chronome have been used in some cases to describe either the molecular mechanisms involved in chronobiological phenomena or the more quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is required.

The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.), (b) in plants (leaf movements, photosynthetic reactions, etc.), and in microbial organisms such as fungi and protozoa. They have even been found in bacteria, especially among the cyanobacteria (aka blue-green algae, see bacterial circadian rhythms). The most important rhythm in chronobiology is the circadian rhythm, a roughly 24 hour-cycle shown by physiological processes in all these organisms. The term circadian comes from the Latin circa, meaning "around" and dies, "day", meaning "approximately a day." Many other important cycles are also studied, including:

* Infradian rhythms, which are cycles longer than a day, such as the annual migration or reproduction cycles found in certain animals or the human menstrual cycle.

* Ultradian rhythms, which are cycles shorter than 24 hours, such as the 90-minute REM cycle, the 4-hour nasal cycle, or the 3-hour cycle of growth hormone production.

* Tidal rhythms, commonly observed in marine life, which follow the roughly 12-hour transition from high to low tide and back.

* Gene oscillations — some genes are expressed more during certain hours of the day than during other hours.