Chronotype variability and patterns of light exposure of a large cohort of United States residents

Chronotype (i.e., disposition for activity early or late in the day) has traditionally been measured with questionnaires. A few studies with small sample sizes have also been conducted using actigraphy devices. In the present study, analysis was conducted of the daily pattern of activity of 1887 United States residents who wore actigraphy devices for a whole week. The devices also recorded the participants’ exposure to light. As determined by cosinor analysis, the mean pattern of ambulatory activity exhibited robust 24- hour oscillation with a peak at 14:48. On average, participants went to sleep 2 minutes before midnight and woke up at 07:43. The distribution of chronotypes (defined as the midpoint of sleep) had a mean of 03:50, and 95% of all chronotypes were between 01:00 and 07:00. The mean duration of exposure to bright daylight was 3.57 hours per day. Duration of daily exposure to bright light was moderately but significantly correlated with chronotype (r = -0.18). The acrophase of the rhythm of exposure to bright light was significantly correlated with chronotype (r = 0.27) and with the acrophase of the activity rhythm (r = 0.36). Chronotype did not vary with the seasons, but exposure to bright light was longer in summer and spring than in winter and fall. These results confirmed chronotype findings from actigraphic studies with smaller sample sizes, endorsed their equivalence to the results of questionnaire studies, and confirmed and extended previous observations that urban dwellers have limited daily exposure to sunlight.