Biological rhythms

Biological rhythms are the repeating cycles of activity which occur in living organisms. The best-known example is the daily or circadian rhythm, which fits the cycle of day and night. 

Most forms of life have natural rhythms which fit them to natural changes in their environment. Animals are most active in part of the day, and rest or sleep at other times. They may be:
 Diurnal, which describes organisms active during daytime
 Nocturnal,  active in the night
 Crepuscular, active during the dawn and dusk hours.

Plants undergo daily cycles as well. Photosynthesis has parts which need light, called light-dependent reactions, and parts which can continue at night (light-independent reactions).

Many other important cycles are also studied,Refinetti, Roberto 2006. Circadian physiology. CRC Press/Taylor & Francis Group.   Lay summary including:
 cycles longer than a day, such as the annual migration or reproduction cycles found in certain animals or the human menstrual cycle.
 cycles shorter than 24 hours, such as the 90-minute REM cycle, the 4-hour nasal cycle,This is the alternating congestion and decongestion of each side of the nasal cavity. or the 3-hour cycle of growth hormone production.
 Tidal rhythms are often seen in marine life, which follow the roughly 12.4-hour transition from high to low tide and back.
 Lunar rhythms, which follow the lunar month (29.5 days). They are relevant for marine life, as the level of the tides is affected by the lunar cycle. 
 Gene oscillations – some genes are expressed more during certain hours of the day than during other hours.

 References 

 Relaate pages 
Homeostasis

Category:Physiology