Ticktock, mice have a gene called Clock.

Even without alarm clocks, people tend to follow a schedule of activity by day and sleep by night, guided by an internal 25-hour clock. Mice, too, have a circadian rhythm, following a 23.7-hour clock that makes them scurry in the dark and snooze in the light.

By monitoring mutant mice for aberrant rhythms, researchers have created a strain of mice with no rhythm at all. They have used those mice to home in on what they call the Circadian Locomotor Output Cycles Kaput, or Clock, gene, reports Joseph S. Takahashi, a neurobiologist at Northwestern University in Evanston, Ill. This is the first circadian gene found in mammals, he and his colleagues report in the April 29 SCIENCE.

"We work from the [behavior] back to the genes," explains Northwestern's Lawrence H. Pinto. For this work, the researchers tested the offspring of mice subjected to gene-altering chemicals. First they exposed the young mice to 12 hours of darkness and 12 hours of light while using a computer to track when the animals rotated the exercise wheels in their cages.

Then the scientists turned off the lights. Over time, 1 of the 304 mice started following a 25-hour clock. Breeding experiments revealed that a single gene led to the longer cycle. Mice that inherit two of these mutant genes lack any circadian rhythm, Pinto says. They still exercise as much as normal mice, but they spread their activity out over an entire day.