this gene is a member of the period family of genes. it is expressed in a circadian pattern in the suprachiasmatic nucleus (scn), the primary circadian pacemaker in the mammalian brain. genes in this family encode components of the circadian rhythms of locomotor activity, metabolism, and behavior. circadian expression in the scn continues in constant darkness, and a shift in the light/dark cycle evokes a proportional shift of gene expression in the scn. per1 and per2 are necessary for molecular timekeeping and light responsiveness in the master circadian clock in the scn, but little data is shown on the concrete function for per3. per3 was found to be important for endogenous timekeeping in specific tissues and those tissue-specific changes in endogenous periods result in internal misalignment of circadian clocks in per3 double knockout (-/-) mice. per3 may have a stabilizing effect on per1 and per2, and this stabilizing effect may be reduced in the per3-p415a/h417r polymorphism.