Twins under shift work: a case study of sleep log data

Two pairs of female twins (one pair monozygotic (MZ) and one pair dizygotic (DZ) attended an evening class at the same college and worked at the same factory under a rotating shift schedule. All the subjects agreed to keep sleep logs for 30 days. Values for rising time were quite similar within each pair especially during the morning-shift schedule, while during afternoon shift and holidays, rising time appeared more similar in the MZ pair, but their bedtime seemed rather different. Morningness-Eveningness score and the amplitude of the circadian sleep and wakefulness (based on an autocorrelation of the data) seemed more similar in the MZ pair. Morningness-Eveningness score is known to be closely correlated with the phase position of subjects endogenous biological rhythms, e.g., body core temperature. Characteristics of genetic control in phase and amplitude of circadian rhythm measured by the above-mentioned procedure might possibly be detectable even under the constraints of shift-work schedule.