·    Circadian rhythmicity is a fundamental property possessed by all organisms.

·   Circadian rhythms can change sleep-wake cycles, hormone release, body temperature and other important bodily functions.

·   Circadian rhythmicity is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave activity, hormone production, cell regeneration and other biological activities. In addition, photoperiodism, the physiological reaction of organisms to the length of day or night, is vital to both plants and animals, and the circadian system plays a role in the measurement and interpretation of day length.

Timely prediction of seasonal periods of weather conditions, food availability or predator activity is crucial for survival of many species. Although not the only parameter, the changing length of the photoperiod ('day length') is the most predictive environmental cue for the seasonal timing of physiology and behaviour, most notably for timing of migration, hibernation and reproduction”

All eukaryotes and some microbes (e.g., cyanobacteria) display changes in gene activity, biochemistry, physiology, and behavior that wax and wane through the cycle of days and nights.Examples:

  • the level of the hormone melatonin that rises in your body during the night and falls during the day.
  • fruit flies (Drosophila melanogaster) hatch in greatest numbers just at dawn.
Even when the organism is placed in constant conditions (e.g., continuous darkness), these rhythms persist. However, without environmental cues, they tend to be somewhat longer or somewhat shorter than 24 hours — giving rise to the name circadian rhythms (L. circa = about; dies = day).The genetics and molecular biology of circadian rhythms have been studied in several model organisms including

  • some unicellular eukaryotes
  • fungi
  • plants (Arabidopsis)
  • invertebrates (Drosophila)
  • mammals (mice, rats, and humans)
What has emerged are some remarkable similarities in mechanisms across these various groups.