Circadian rhythms are found in most organisms. They are endogenous rhythms of activity, hormone production or gene expression with a cycle length of approximately 24 hours. A central clock, whose principle role appears to be co-ordinating internal rhythms with the daily rhythm of day and night (Figure 1) controls the different observed rhythms in a single organism. In animals, fungi and cyanobacteria, this circadian clock is formed from loops of negative regulation of gene and protein expression1. Whether this is also true of plant clocks is the subject of some current debate.

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