Latitude and social habits as determinants of the distribution of morning and evening types in Brazil

Previous application of a Portuguese Version of the Horne and Ostberg (H&O) questionnaires in Brazil showed a displacement of the distribution of Morn ingness/Eveningness (M/E) types towards morningness, demanding a reclassifi cation of the score's limits for each of the 5 possible M/E types. Among ci ty, age and sex, city was the only factor determining significant group dif ferences. In the present paper, the same version of the H&O's questionnaire was applied to a distinct population of 260 adults, balanced according to age, sex and city. Data were collected from October to December, to control the effect of the photoperiod. City remains the only factor determining si gnificant group differences (p<0.001). Moreover, there does not appear a co herent latitude trend that could explain differences among cities as a func tion of photoperiod. Local social habits could probably explain our results , suggesting further investigations of peculiar temporal patterns.