Chronobiology: biological clocks and rhythms of the skin.

The cyclicity of time affects virtually all aspects of our being and is the basis of the underlying rhythmicity which is typical of our lives. To 'tell time', most living organisms use internal timing mechanisms known as 'biological clocks'. These 'clocks' coordinate our physiological and behavioral functions and interactions with our environment. One of the strongest influences on rhythmicity is the solar day. The study of these temporal rhythms in biological systems has been coined chronobiology. With the present article we aim to give an overview on chronobiology. Examples of chronobiological effects on skin will be described. Particular emphasis will be placed on circadian rhythms (including rhythms that take place within a 24-hour period, including so-called infradian and/or diurnal rhythms) but also on seasonal variations (circaannual rhythms).