ABSTRACT
Earth is not a static environment, since there are cyclic changes occurring all the time. Some of these environmental changes can be predictable, when they are associated with geophysical cycles, or unpredictable, when related with a meteorological phenomenon (such as rain). Environmental cycles occur within different periodicities: annual periodicity in association with the rotation of the Earth around the Sun, appearing seasons in temperate regions; lunar periodicity associated with the rotation of the moon around the Earth; and finally, daily periodicity associated with the rotation of the Earth on its axis, originating the daily alternation of light and darkness. All these cycles acting on living organisms through evolution, promoted the appearance of molecular oscillators or biological clocks, which can be used by animals to keep time and anticipate recurrent events, in order to optimize their survival. Such a multioscillatory pacemaker, which endogenously controls biological rhythms, is located in the hypothalamus, pineal organ and eyes of vertebrates.
