THE CIRCADIAN CLOCK: ORCHESTRATING GENE EXPRESSION AND PHYSIOLOGY

Timing is everything. How true is this proverb? Time has neither a beginning nor an end, and hence, it is difficult to define. What we experience as time is related to a reference point and, hence, relative. Living on earth has made us use the sun as reference and the 24-hour succession of light and darkness is probably the most pervasive epi-genetic influence in the evolution from a single cell organism to man. This periodic succession of light and darkness provided the base for relative timing of biological processes over the 24 hours of a day. Because energy supply is the limiting parameter for survival, a system for optimal timing of energy expenditure and uptake developed. The mechanism of this system took the shape of a cycle reflecting the recurrence of sunrise and sunset, and is termed a «circadian clock » – a clock with a period of about one day (latin: circa diem). The inter-nalization of environmental time within the organism not only allows organization of biological processes along the 24-hour time scale bu