In order to know when and how infants obtain their circadian sleep-wake rhythm, infants' developing circadian rest-activity rhythm and mothers' circadian rhythm in the postpartum period were examined using actigraph monitoring. The subjects were 11 primipara and their infants. Actigraphic recordings for the infants and their mothers were made over 3-5 continuous days during the 3rd, 6th, 9th and 12th weeks after birth. A 24-h peak on a mean autocorrelogram of the infants' movements was detected at the 3rd week. The infants' circadian rest-activity rhythm already existed in the 3rd week. The amplitude of this 24-h peak gradually increased from the 6th to 12th week. This may be useful as an index of the development of infants' circadian rest-activity rhythm. An 11-h peak was also observed at the 3rd week. This 11-h peak was thought to be a semi-circadian rhythm. Regarding the mothers, the amplitude of the 24-h peak on the mean autocorrelogram at the 3rd week was the smallest of all other weeks, and it became larger from the 3rd to 12th week. This meant that the mothers' circadian rhythm at the 3rd week was influenced by their interrupted sleep at night to take care of their infants. The mother-infant synchronization is probably the 1st factor in the entrainment of infants' circadian sleep-wake rhythm. In this study, we also propose a novel method for compensating for missing data in autocorrelogram analysis.
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