Life on earth is shaped by multiple environmental cycles. One of the most influential is the succession of day and night. In most species – from procaryotes to humans – internal timekeepers (so called circadian clocks) have evolved that anticipate these daily events and fine-tune physiology to the varying demands of activity and rest. Circadian rhythms are almost omnipresent and cover all aspects of biology from behaviour down to cell cycle control and chromatin modulation. In mammals, a master clock in the suprachiasmatic nuclei (SCN) of the hypothalamus is synchronised to geophysical time via visual and non-visual photoreceptive systems in the retina. From the SCN, time information is transmitted to numerous peripheral clocks located throughout the body. more...