A. All animals, including humans, experience daily ups and downs in physiological processes, such as body temperature and hormone secretion, as well as in behaviors such as sleeping and waking. Known as circadian rhythms, these patterns are governed by a core "clock" in our brains that's primarily tuned to the 24-hour cycle of light and dark.

Being in sync with night and day is so critical that animals have evolved to sense light in at least two ways: with the eyes and with photoreceptive proteins in the brain, says UW-Madison genetics professor Jerry Yin. Still, big differences do exist among people in how this system operates.

For example, some people's rhythms are substantially longer or shorter than 24 hours, so that "they don't quite line up with the sun, and are constantly catching up or falling behind," Yin says. This occurs when mutations are present in the 10 or so genes that control the core clock.

In fruit flies, scientists have also discovered that the clock is subdivided into groups of "morning" and "evening" cells, which control daily periods of activity and inactivity by taking turns acting as master and slave.

"So another way you could possibly get morning or evening people is if one of those two areas dominated more than the other," Yin says.

-- Produced in cooperation with University Communications

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