Biological clock

A biological clock is a physiological mechanism which regulates the timing of any biological process or activity. The biological clock is not a single cell, neuron or collection of neural cells. Neither is it a result of activity of a single gene. Instead, several mechanisms have been identified in various animal groups. Often the biological clock refers to a cellular oscillator in vertebrates, residing mostly in the subprachiasmatic nucleus (in mammals), controlling the diurnal (or day-night) cycle of animal activity, wakefulness, hormonal cycles ets. Other, long terms neural oscillators control the reproductive cycle, whose lenght varies a lot between species. Most biological clocks are regulated by light in some manner, either by means of phase locking to the light-dark cycle (as in the diurnal oscillator) or some other regular change (seasonal, lunar). Otherwise, i.e. colloquially, a biological clock generally refers to the onset of menopause and associated loss of reproductive capabilities in female humans.