Mice are nocturnal. When both wild type and Chrono knockout mice are kept in an environment with 12 hours of light (blue) and 12 hours of dark (white).They align their activity (green) to the lighting schedule and run in the dark. When the animals are switched to constant darkness (arrow), the influence of their internal circadian clock is revealed. Both wild type and Chrono knockout mice maintain regular patterns of activity and rest that are slightly shorter than 24 hours -- each successive day the onset of activity occurs somewhat earlier. However, the endogenous activity period of Chrono knockout mice is about 25 minutes longer than the period of wild type mice.
Ron Anafi, M.D., Ph.D.; John Hogenesch, Ph.D., Perelman School of Medicine, University of Pennsylvania