Circadian time keeping: the daily ups and downs of genes, cells, and organisms

Approximately 24-h cycles are produced by circadian clocks in light-sensitive organisms, from cyanobacteria to humans.

• Circadian clocks operate independently of external time cues.
• Recent experiments suggest that protein phosphorylation cycles may be fundamental timekeeping mechanisms in certain organisms.
• Mammalian body cells contain self-sustained circadian oscillators that continue functioning autonomously in tissue culture.
• A central pacemaker coordinates these oscillators in mammals.
• Circadian timing influences various physiological processes, including rest-activity cycles and metabolism.

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