As expected for the general adult population our study

As expected for the general adult population [19], our study showed that 92% of the subjects had a monophasic pattern. The monophasic sleep pattern remains during adulthood, except in some societies that have the habit of napping after lunch [20]. Several factors could determine the ideal total sleep duration, if it can be achieved with a single sleep episode (monophasic pattern) or with more than one (biphasic or polyphasic). First, there is an individual propensity that determines the ability of a subject to sleep at different times [21,22]. Moreover, social factors could determine the need to perform various sleep episodes [23–26]. Biphasic and polyphasic sleep patterns, represented by 8% of the evaluated population, may be associated with the ability of humans to fall asleep several times a day. This ability to fragment sleep within a 24-hour period may be an adaptive strategy in individuals who cannot have the desired amount of sleep [27]. Our data also showed that most of the subjects were defined by the actigraph as morning type. Other researchers observed an increase in morning type subjects in the general population. An epidemiological study conducted in the adult population of New Zealand, using MEQ scores, showed that 49.8% of the population was characterized as morning type [3]. Another study that evaluated 566 adult subjects, without sleep complaints, found that 62.1% were morning type based on the MEQ score [28]. One can speculate that light exposure would be an environmental factor that is associated with the genetic predisposition to the phenotypic ras inhibitor of morningness and eveningness [8]. In São Paulo, the difference between the shortest and the longest day throughout the year is approximately two hours, whereas in London this difference is eight hours. However, in a study performed in France [28], similar proportions of morningness were observed. Thus, other factors seem to influence this increased frequency of morningness in São Paulo city. We found that the morning type group had higher TST (“main episode” and “24h”) than the other groups and were also more common among those who slept more than six hours per day. These data corroborate the study by Taillard and colleagues [29], which showed that evening type subjects had shorter time in bed and increased need for sleep. We suggest that evening type individuals could experience consequences associated with chronic sleep deprivation. Our study showed that the mean age of the evening type subjects, identified by the cluster analysis, was lower than that in the other groups, which is consistent with other studies that showed a trend for increased morningness with age [30], even after correcting for socioeconomic and demographic factors [3,31]. The main limitation of our study was the difficulty of sampling the general population of São Paulo with the use of the actigraph. Our sample of subjects who wore an actigraph was comparable to the general population in terms of the male/female proportion and age, but significant differences were found for the socioeconomic status. Furthermore, from the sample who agreed to wear the actigraph, many subjects were excluded from the analysis, mainly due to failure in the consecutive days of use. This may represent the difficulty of compliance in wearing the actigraph despite the time spent training the subjects. As expected in a population-based survey [11], the evaluated sample of subjects is heterogeneous and the health, lifestyle or occupational issues were not considered in this study. Moreover, MEQ was applied by trained interviewers. This could have some bias since the MEQ is a self-reported questionnaire.
Acknowledgments
This study was supported by grants from the Associaçao Fundo de Incentivo a Pesquisa (AFIP) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (#07/50525-1 to RS-S and #98/14303-3 to ST). ST and LB received fellowships from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). LM received fellowships from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.