Recent discoveries about circadian rhythms, the daily oscillations in biological function orchestrated by the brain’s internal clock, are driving new applications that range from better cancer treatment to helping future astronauts adapt to the length of a day on Mars.

The circadian system is best known for governing the sleep-wake cycle: It is the internal clock that makes some people morning larks and others night owls. When it is out of synch, as it is in some sleep disorders, jet lag, or shift work, sleep patterns are disrupted along with all sorts of sleep-dependent behaviors, including cognitive performance and overall functioning.

As it turns out, regulating sleep is only the tip of the circadian iceberg. It is now clear that signals emanating from the brain’s master timekeeper ultimately modulate fundamental physiological activity throughout the body, affecting how much we eat, how drugs affect our liver, and how tumors grow, among other things. That’s because clocks are now known to exist throughout the body.

“For a long time it was thought that the circadian clock was strictly a brain thing,” says Carla B. Green, who studies the circadian system at the University of Virginia. “It’s only fairly recently that we’ve understood that lots of different cells have clocks, including fibroblasts, intestinal cells, liver cells, skin cells, and even cancer cells. That was not appreciated before.”

These new understandings have far-reaching implications for disease prevention and treatment and are increasingly being applied to the design of therapeutic regimens that optimize drug delivery in accordance with circadian timing. In the latest twist, NASA is tapping the minds of circadian researchers to find a way for future space explorers to reset their body clocks in synchrony with the longer-than-24-hour Martian day. (See "Headed to Mars? Pack Bright Lights ").