Young Adults' Sleep Duration on Work Days: Differences between East and West

Abstract

Human sleep schedules vary widely across countries. We investigated whether these variations were related to differences in social factors, Morningness-Eveningness (ME) preference, or the natural light-dark cycle by contrasting the sleep duration and timing of young adults (age: 18-35 years) on work and free days in Singapore (n = 1898) and the UK (n = 837). On work days, people in Singapore had later bedtimes, but wake times were similar to the UK sample, resulting in shorter sleep duration. In contrast, sleep duration on free days did not differ between the two countries. Shorter sleep on work days, without compensatory extra long sleep hours on free days, suggest greater demands from work and study in Singapore. While the two samples differed slightly in ME preference, the associations between eveningness preference and greater extension in sleep duration as well as delays in sleep timing on free days were similar in the two countries. Thus, differences in ME preference did not account for the differences in sleep schedules between the two countries. The greater variability in the photoperiod in the UK was not associated with more prominent seasonal changes in sleep patterns compared to Singapore. Furthermore, in the UK, daylight saving time did not alter sleep schedules relative to clock time. Collectively, these findings suggest that differences in social demands, primarily from work or study, could account for the observed differences in sleep schedules between countries, and that in industrialized societies, social zeitgebers, which typically involve exposure to artificial light, are major determinants of sleep schedules.

Keywords: Morningness–Eveningness preference; free days; natural light; sleep duration; sleep timing; social factors; work days.

Figure 1

Distributions of sleep duration and timing on work and free days in Singapore and the UK. (A) Sleep duration, (B) bedtime, (C) wake time, and (D) mid-sleep time for Singapore and the UK are shown in black and white bars, respectively. Data on work and free days are respectively plotted in the upper and the lower panels.

Figure 2

Distributions of ME preference in Singapore and the UK. The distributions of the total scores in the reduced Morningness–Eveningness Questionnaire (rMEQ) in Singapore and the UK are indicated, respectively, by the black and the white bars. Background is shaded based on the cutoff scores for the extreme, the moderate, and the neither types.

Figure 3

Effects of ME preference on sleep on work days and free days in Singapore and the UK. Regression lines for the effects of the reduced Morningness–Eveningness Questionnaire (rMEQ) score on (A) sleep duration, (B) bedtime, (C) wake time, and (D) mid-sleep time on work days (dashed lines) and free days (solid lines) for Singapore and the UK are plotted, respectively, in the left and the right panels.

Figure 4

Variation in the duration and timing of scotopic period and sleep in Singapore and the UK across a year. Daily value of (A) the duration of scotopic period, (B) dusk time, (C) dawn time, and (D) mid-scotopic time in 2012 in Singapore (solid lines) and the UK (dashed lines) are shown in the left panel. Shaded areas indicate the daylight saving period in the UK. The monthly averages of (A) sleep duration, (B) bedtime, (C) wake time, and (D) mid-sleep time on work days (open symbols) and free days (filled symbols) for Singapore and the UK are plotted in the middle and the right panels, respectively. Considering the possible effects of daylight saving in the UK, we divided the daylight saving period in 2012 (25 March to 27 October) into seven roughly equal intervals and for each, derived the average sleep duration and timing. Similarly, we computed the monthly averages of the sleep variables for each of the five roughly equal intervals during standard time.