Dependence of nighttime sleep duration in one-month-old infants on alterations in natural and artificial photoperiod

Abstract

Human sleep-wake cycles are entrained by both natural and artificial light-dark cycles. However, little is known regarding when and how the photoperiod changes entrain the biological clock after conception. To investigate the dependence of sleep patterns in young infants on the natural and artificial light-dark cycles, 1,302 pairs of one-month-old infants and their mothers were asked to answer a questionnaire. Birth in spring, longer daytime sleep duration, early/regular light-off times, and longer maternal nighttime sleep duration were identified as independent variables for longer infant nighttime sleep duration in both univariate and multivariate analyses. Longer maternal nighttime sleep duration was dependent on shorter naps and early/regular bed times but not on the season. We found that nighttime sleep duration depended on both natural and artificial diurnal photoperiod changes in one-month-old infants. Although sleep patterns of infants mimicked those of their mothers, nighttime sleep duration depended on the season, and was positively associated with daytime sleep duration, only in the infants. These specific variables, which render sleep patterns of the infants different from those of their mothers, might be a clue to reveal the covert acquisition process of mature circadian rhythms after birth.

Figure 1. Distribution of nighttime and daytime sleep duration.

Histograms depicting sleep durations of infants (a,b) and their mothers (c,d) during the nighttime (20:00 to 8:00 h; a and c) and daytime (8:00 to 20:00 h; b and d).

Figure 2. Relationships between nighttime and daytime sleep durations of infants and their mothers.

Binned scatter plots with regression lines and their 95% confidence intervals depicting relationships between nighttime and daytime sleep durations of infants (a), nighttime sleep durations of infants and their mothers (b), and nighttime and daytime sleep durations of mothers (c). Nighttime sleep duration in infants was positively associated with both their daytime sleep duration (a; p < 0.001) and maternal nighttime sleep duration (b; p < 0.001), whereas longer maternal daytime naps were negatively associated with their nighttime sleep duration (c; p < 0.001).

Figure 3. Dependence of nighttime sleep durations on the season and sleep schedule.

Box plots depicting the dependence of nighttime sleep durations on the light-off/bed time (a,b) and season of birth/delivery (c,d) for infants (a and c) and their mothers (b and d). Nighttime sleep duration in infants with irregular light-off times was shorter than those with regular early (p < 0.001) or regular late (p = 0.017) light-off times (a). Nighttime sleep duration for mothers with irregular bed times was shorter than that for those with regular early or regular late bed times (b; both p < 0.001). The duration of nighttime sleep in infants born in spring was longer than those born in autumn (c; p = 0.002), whereas no seasonal difference was found in maternal nighttime sleep duration (d). Solid lines, median; boxes, lower and upper quartiles; whiskers, minimum and maximum; circles, outliers; stars, extreme outliers.