Most organisms, including humans, show daily rhythms of about 24 hours in physiology, hormonal function, and behavior. In mammals, these rhythms are controlled by an endogenous circadian pacemaker localized in the suprachiasmatic nucleus (SCN) of the hypothalamus that determines the temporal organization of several behaviors and physiological processes. Circadian control of daily rhythms differs in diurnal and nocturnal species but many of the mechanisms that may explain these differences remain still unknown. The aim of this review is to summarize our current knowledge of the circadian clocks and the differences between diurnal and nocturnal species