Is circadian type associated with sleep duration in twins?
We used the community-based University of Washington Twin Registry to investigate the genetic association between circadian type and sleep duration. Habitual sleep duration was obtained by self-report and circadian type was measured using a reduced 5-item Horne–Östberg Morningness–Eveningness questionnaire. Univariate and bivariate genetic analyses were fit using structural equation models. We used multinomial logistic regression to examine the overall and within-pair effects of circadian type on sleep duration in total and monozygotic (MZ) restricted twin samples. We surveyed 1620 twins, mostly female (70%), from same-sex pairs (1098 monozygotic, 522 dizygotic) with a mean age of 36 years (SD = 15). Among all twins, 23% were habitual short (<7 h/night), 68% normal (7–8 h/night), and 9% long (≥9 h/night) sleepers. Overall, 33% were morning-type, 52% neither-type, and 15% evening-type. The heritability of sleep duration was 31% and 40% for circadian type (both P < 0.05). The bivariate analysis did not reveal shared genetic influences on circadian type and sleep duration. When analyzing twins as individuals, eveningness was associated with short and long sleep duration in the total (P < 0.001) and MZ restricted (P < 0.05) twin samples. These findings were attenuated in the within-pair analysis in both samples suggesting that familial factors, namely genetics and shared environment, are in the pathway of interest or confound the association between circadian type and sleep duration. Because both short and long sleep is associated with adverse health outcomes, our findings suggest circadian evening-type may represent an endophenotype for poor health.
Is circadian type associated with sleep duration in twins? Sleep Biol Rhythms. 2012; 10:61-8