  Background: We propose a method for estimating the timing of in-bed intervals
using objective data in a large representative U.S. sample, and quantify the
association between these intervals and age, sex, and day of the week. Methods:
The study included 11,951 participants six years and older from the National
Health and Nutrition Examination Survey (NHANES) 2003-2006, who wore
accelerometers to measure physical activity for seven consecutive days.
Participants were instructed to remove the device just before the nighttime
sleep period and put it back on immediately after. This nighttime period of
non-wear was defined in this paper as the objective bedtime (OBT), an
objectively estimated record of the in-bed-interval. For each night of the
week, we estimated two measures: the duration of the OBT (OBT-D) and, as a
measure of the chronotype, the midpoint of the OBT (OBT-M). We estimated
day-of-the-week-specific OBT-D and OBT-M using gender-specific population
percentile curves. Differences in OBT-M (chronotype) and OBT-D (the amount of
time spent in bed) by age and sex were estimated using regression models.
Results: The estimates of OBT-M and their differences among age groups were
consistent with the estimates of chronotype obtained via self-report in
European populations. The average OBT-M varied significantly by age, while
OBT-D was less variable with age. The most pronounced differences were observed
between OBT-M of weekday and weekend nights. Conclusions: The proposed
measures, OBT-D and OBT-M, provide useful information of time in bed and
chronotype in NHANES 2003-2006. They identify within-week patterns of bedtime
and can be used to study associations between the bedtime and the large number
of health outcomes collected in NHANES 2003-2006.
