Diurnal preference for sleep-wake timing and activity is well known and can be quantified by simple questionnaires. A consistent finding across different studies is that there tends to be increased morning preference in women compared to men of same age. About 50% of variance in diurnal preference is heritable, so it should be expected that differences in diurnal preference should be associated with underlying genetic variation. A primate-specific, variable number tandem repeat (VNTR) polymorphism in the coding region of PER3 is associated with diurnal preference and also delayed sleep phase disorder. Diurnal preference is a complex/heterogeneous phenotype that shows high level of heritability. It is directly related to intrinsic circadian function and determined by the interaction of the circadian pacemaker and the sleep homeostat. Roughly half of the variance observed in diurnal preference is heritable while the remainder is presumably determined by environmental factors.