Biological rhythms are the repeating cycles of activity which occur in living organisms. The best-known example is the daily of 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
Many other important cycles are also studied, 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, 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.