ABSTRACT
In mammals, this direct protective function has evolved into a hormonal signal. Not light, but darkness, is the stimulus for secretion in the pineal gland, which is the main organ of melatonin synthesis. The melatonin concentration in the blood oscillates in a circadian manner, with peak concentrations in the dark. In mammals, melatonin is the most important synchronizing signal of circadian and circannual rhythms [2-4].
