There is a pattern to when we wake up and get tired over a 24-hour period, scientists call it the circadian rhythm. Most other animals experience it too.

As winter arrives with its shorter days, grizzly and black bears begin their long naps, called hibernation. During this time bears will still move about a bit, but they don’t eat while hibernating.

Scientists at Washington State University were curious how grizzly bears’ circadian rhythms are affected during hibernation and conducted a study. What they found was although the animals are much less active, the energy their bodies produce still grows and fades over the course of a day as if they were awake, but at a much lower level.

Co-authors on the study include WSU graduate student and first author Ellery Vincent as well as Blair Perry (biological sciences) and Charles Robbins (environment and biological sciences), and Joanna Kelley of University of California, Santa Cruz. This research received support from the National Science Foundation and the Bear Research and Conservation Endowment at WSU.

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