A plant spreads its leaves, an insect scurries, a mold produces spores, and human blood pressures, hormone levels, and temperatures ebb and flow, in an oscillating pattern throughout the day. Animals mate and molt, migrate and hibernate, to the beat of internal timekeepers in sync to predictable changes in the environment. Researchers are exploring these biological clocks in a variety of species.

Mice: Some of the rodent residents of Joseph Takahashi's neurobiology lab at Northwestern University exercise at odd times of the day. Those with one mutant copy of the gene clock follow a 25-hour day, while those with two mutant genes follow a 26- to 29-hour day for two weeks, then simply stop cycling.

The photosynthetic set: At the University of Virginia, associate professor of biology Steven Kay and postdoctoral research associate Andrew Millar scan transgenic seedlings of the mustard weed Arabidopsis for those that glow to the beat...

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