Many of the biological activities of living organisms are light dependent and revolve around the solar clock. A variety of behaviours and physiological processes are under the control of internal biological clocks. These internal clocks are remarkably synchronized with the external solar clock (day-night cycle of 24 hour periodicity). The molecular mechanisms of biological clocks have recently been uncovered. Although there are several molecules, which have a key role to play in the functioning of biological clocks, cryptochromes have been implicated to be of special importance. A detailed study of cryptochromes may provide some answers to problems related to the abnormal functioning of biological clocks in humans.