[Rhythms, XXXX and light].

@entity1 @entity308 @entity7383 @entity2493
Many aspects of life in plants, animals and @entity1 are controlled by light. Endogenous, so-called circadian rhythms in the body deviate from the exact 24-hour day and have typically a period of around 25.5 hours in @entity1 . Normally these rhythms adapt to the external 24-hour day-and night changes but under constant conditions the rhythms can free run. Many studies show how important the interplay between light and the circadian rhythms are for @entity1 as well as for other organisms. The control of these rhythms by light is mediated via the retina and the @entity2493 system in @entity1 . The adaptation of the rhythms is very important in shift work, in rapid jet lag travels over time zones, etc. Organisms often use the circadian rhythm to determine the length of day and of night, a feature that has given rise to the term biological clocks. A biological clock provides possibilities to determine the proper time for physiological processes to start in plants and animals (flowering, hibernation etc). The importance of light and circadian rhythms for @entity7383 and @entity308 is also discussed. For several organisms one has now been able to specify genes that determine the period of the clocks. The rhythmic physiologic processes, the light reactions and the general importance of light for rhythms and for @entity1 are now studied at the molecular level.
