Genes involved in melatonin synthesis, such as Aa-nat, may be important for our understanding of diurnal preference and circadian rhythm disturbances in humans. In Japan, Hohjoh et al. reported increased allelic frequencies of the 619A allele of the G619A Aa-nat gene polymorphism in a sample of Delayed Sleep Phase Syndrome (DSPS) patients. The present study sought to analyze G619A polymorphism frequency in a Brazilian sample, including DSPS patients. We found almost no allelic variation for G619A polymorphism in our sample, except for two heterozygous samples out of 551. Our results leave open the question of whether there would be an association if there were some genetic variation in our population. It is important to analyze different ethnic groups in order to validate the effect of G619A polymorphism on sleep timing.