Circadian clocks—the biological timekeepers that operate on a daily cycle—keep virtually every living creature in tune with its environment. These internal clocks regulate a wide range of fundamental biological processes, including movement, smell, sleep, mating, and feeding. A true circadian clock is endogenous; that is, it keeps time even in the absence of external cues. The clock can, however, be reset, or entrained, by daylight, allowing the synchronization of circadian rhythms to the phase and period of environmental light and dark cycles.
That these clocks exist is well established, but discerning how they work is another matter entirely. Clock researchers have found that the most fertile ground in which to study circadian rhythms is the fruit fly, Drosophila melanogaster. The step-by-step discovery of Drosophila clock genes, driven largely by screening mutant flies, has pushed forward the identification of corresponding mammalian clock genes.
Most researchers believe that the basic clock...
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