Chronobiology is the study of biological clock
. There are different types of biological rhythm including circadian rhythms
which are 24 hours long, ultradian rhythms which are shorter than 24 hours and infradian which are longer than 24 hours.
The sleep/wake cycle in humans and leaf movement in plants are examples of circadian rhythms. The rate of an animal's heartbeat is an example of an ultradian rhythm. The menstrual cycle and the seasonal reproductive cycles in animals and plants are examples of infradian rhythms.
Biological clocks which control these rhythms are free-running under constant conditions and can be reset by light, which is particularly handy if you fly to a different time-zone and want to adjust your sleep/wake cycle. They are implemented by photoreceptor proteins which change properties upon light absorption and through association with different proteins and/or genes either inhibit or promote gene transcription. The rhythm of a cycle is determined through repeated transcriptional feed-back loops. One of the unique features of biological clocks is the temperature compensation ability whereby a cycle remains a constant length regardless of environmental fluxes in temperature. The rate of all biochemical reactions are subject to kinetic parameters which means that generally an increase in temperature increases the rate of a reaction. The mechanism by which biological clocks compensate for changes in temperature is currently unknown although it is thought that the intricate interractions between transcriptional feed-back loops is a key feature.
All organisms feature biological clocks althought the exact nature and biochemical composition of these clocks vary significantly. These cycles allow the temporal separation of incompatible biological events which is particularly important in unicellular organisms and in plants utilising the C4 metabolic pathway
All information taken from lectures from Professor P. M. Gilmartin at the University of Leeds, UK.
Additional references for furthur detail:
Molecular Bases of Circadian Rhythms. Harmer, SL, Panda, S & Kay SA
Annual Review of Cell and Developmental Biology (2001) 17:215-253
Time zones: A Comparative Genetics of Circadian Clocks. Young, MW & Kay SA
Nature Reviews Genetics (2001) 2 702-715