Brain circadian clocks timing the 24h rhythms of behavior.

The mammalian circadian system is composed of multiple cellular biological clocks synchronized among each other, maintaining thus robust rhythms of behavior. The hypothalamic suprachiasmatic nucleus, or SCN, harbors the principal clock that coordinates circadian rhythms in peripheral tissues and brain structures. Besides the SCN, other central regions contain 24 h activity in electrical properties, metabolic processes and gene expression. Few of them show robust and self-sustained oscillatory activity, but others need the SCN to keep a prolonged rhythmic profile. Thereby, if the SCN is the main circadian organ in mammals, what is the biobehavioral relevance of a circadian brain with multiple clocks? The present review is an overview of the currently known brain circadian system in mammals; highlighting those structures characterized as circadian clocks and their potential role for the control of rhythmic behaviors, and the functional relationship with the SCN. Even if the SCN remains the principal central clock for the control of behavioral rhythms, a multiple brain circadian clock network might be much more advantageous for the organism, facilitating the behavioral and physiological adaption to the rhythmic environment, keeping thus a healthy status.