Circadian rhythms are regulated by clock proteins through post-translational modifications. Indeed, Casein kinase I epsilon (CKIvarepsilon) promotes reversible phosphorylation of PER proteins, and a deficiency in this phosphorylation has been implicated in human sleep disorders. Here, we investigated the CKIvarepsilon S408N polymorphism in a Brazilian population sample. The N408 allele was previously described to be much less frequent in individuals with Delayed Sleep-Phase Syndrome (DSPS), than in the general Japanese population, suggesting a protective function for the allele against the disease. We found that this polymorphism is very rare in the Brazilian population (1.37%), indicating that it has no influence on susceptibility to circadian rhythm sleep disorders. Therefore, it is necessary to account for adaptative influences in genetic background, analyzing different groups with different photoperiods, to validate the effects of this and other polymorphisms on sleep and circadian disorders.