Protein may help the eyes tell time

The light-sensitive skin cells of the African horned frog may have resolved a lingering mystery about the internal biological clock that all mammals possess, suggests a new study.

This physiological timepiece, whose period is almost exactly a day, governs many bodily rhythms. To synchronize with the day-night cycle of the outside world, the mammalian biological clock adjusts itself daily by sensing sunlight and other stimuli.