Genetic and environmental influence on morningness–eveningness

Little is known about the causes of the wide variability in circadian rhythms in humans. We present the first report of genetic influence on a circadian phenotype, the morningness-eveningness disposition. Two hundred and five pairs of adult monozygotic reared-together twins, and 55 pairs of adult monozygotic and 50 pairs of adult dizygotic reared-apart twins completed a morningness-eveningness questionnaire. Biometrical model-fitting showed that genetic variability accounted for about 54% of the total variance in morningness-eveningness; age contributed to 3% of the variance; and the remaining variance was explained by nonshared environmental influences and measurement error. Shared rearing environmental effects were not significant. The husband-wife correlation for morningness-eveningness was 0.25. This correlation appears to be due largely to assortative mating (the tendency of like to marry like) at the time of marriage rather than a consequence of cohabitation during marriage.