Abstract
Circadian rhythms develop from an ultradian to a circadian rhythm during a few months in the early human life stage. One of the strong factors in promoting the development of circadian rhythms during infancy is maternal rest-activity rhythms. However, few studies have examined comparing the rest-activity rhythms of parents and infants. This study aimed to describe longitudinal changes in the rest-activity rhythms of a family at seven time points, from late pregnancy to eight months after birth. Data on a seven-day activity was obtained from Actigraphs (Micro-mini-RC, Ambulatory Monitoring Inc., NY, USA) and analyzing activity data using Action-W4 software (ver. 3.10.0.3, Ambulatory Monitoring Inc.). Double plot diagrams based on the three individuals' average activity levels were created and visually observed longitudinal changes. Also, periodic parameters, midline estimated rhythm (mesor), amplitude, and 24-hour autocorrelation coefficient, were calculated. This study was approved by the Ethics Committee (approval number: U2024-007). The participants were a mother (38-year-old primipara), a father (34 years old), and a male infant. The male infant was delivered vaginally at 40 gestation weeks (GW). The mother and newborn were discharged from the hospital on day 6 after birth and slept in the same room with the father. Both parents took childcare leave; one year for the mother and one month for the father, respectively. The mesor and the amplitude were both lowest at one month after birth and increased with age. When the difference in the activity counts between active and inactive periods increases, the rest-activity rhythm becomes clearer. The mesor and amplitude, the parameters indicating these differences, show the rest-activity rhythms changing robustly. The 24-hour autocorrelation showed distinctive trends for the father and dyad of mother and infant. The father's value was 0.4-0.6, maintaining regularity of the rest-activity rhythm. The mother's value decreased to 0.3 at 1 week after birth, approaching the value of the infant, and then the changes gradually approached the father's value throughout the study period. Also, the values of the mother and child were synchronized, and the mother's value was 0.1-0.3 higher than the child's value at each measurement point. The regularity of rhythm as indicated by the mesor, amplitude, and 24-hour autocorrelation value reached its lowest value one week after birth among the parents and infant, which means that the periodicity of the rhythm was impaired. One month later, the regularity of the rhythm increased, and the father maintained a high value. The changes in the values of the mother and infant were synchronized, with the mother's value always remaining high. It is possible that the regularity of the father's rhythm contributed to the establishment of a rest-activity rhythm in the mother and infant.