Circadian rhythm of sleep and wakefulness : estimation of genetic influences with the method of twin study

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Individual differences in a phase of circadian rhythm has been investigated in connection with adaptability for shift work. In the last decade, circadian rhythm sleep disorders, which shows abnormal circadian phase and/or rhythmicity, are frequently reported. Although the importance of the individual differences in circadian rhythms are well recognized, it is still unclear why the individual differences arise, in other words, genetic and environmental effects on the human circadian rhythm had not been well investigated. Recently Linkowski and co-workers investigated human circadian variations in cortisol secretion or in cardiovascular functions. They suggested genetic influences on several parameters of human circadian rhythm, however, sleep and wakefulness rhythm itself was not dealt with. This study aimed at evaluating genetical influences on several circadian rhythm parameters of sleep and wakefulness with the method of twin study.Twenty two pairs of twins (11 monozygotic and 11 diz … More ygotic twin pairs) participated in this study. Twin zygosity was determined by means of a questionnaire. They were asked to log their sleep on the daily basis for at least 7 days. They also asked to fill out the questionnaire concerning their habitual pattern of sleep and wake behavior and Morningness and Eveningness Questionnaire (Horne and Ostberg, 1976). Based on the sleep logs, the author scored each 15 min.unit of sleep as 'one' and that of wakefulness as 'zero.' Autocorrelation function was employed to estimate the amplitude of circadian component of their sleep and wakefulness cycle.Intra-class correlation coefficients for mean bedtime and rising time were not different between the monozygotic and dizygotic twin pairs, while that for circadian amplitude, estimated by autocorrelation function, and mornignness and eveningness score, which is considered to be correlated with core body temperature, were higher in the monozygotic twin pairs. The latter two variables seem to be genetically determined. Less