Internal timing system

The circadian system provides an endogenous daily programme for the entire organism and may fulfil even the role of a calendar. It is

an integrating system: it encompasses a central clock and clocks in peripheral organs which are coordinated and synchronized by the

central clock. At the same time the circadian system depends on its environment and is periodically entrained by surrounding stimuli,

mostly by photic ones. The discipline which studies the role of time in biology, the so called chronobiology, covers mostly the circadian

rhythmicity and affects many branches of theoretical medicine like physiology, biochemistry, molecular biology, neurobiology. It intervenes,

however, also in clinical disciplines-internal medicine, endocrinology, occupational medicine, neurology, psychiatry, oncology,

etc. It affects almost everything. Therefore, it is necessary to pay attention to it and to understand it. Chronobiology affects even each

of us in our ordinary life. Its understanding leads us to awareness of the necessity to keep a regular daily schedule in order to perform

our everyday tasks at the appropriate time for them so that they could not stress us.

Keywords: circadian system, biological clock, SCN, peripheral clocks, clock genes, melatonin, circadian disorders