This is the first editorial in Sleep and Biological Rhythms. As the first topic, we have chosen to describe a study of a Japanese birth cohort. Beginning in 2011, the Ministry of the Environment has conducted the Japan Environment and Children’s Study (JECS), a large-scale nationwide epidemiological cohort study involving 100,000 mother–child pairs living throughout Japan [1]. The JECS has published 45 reports, including two on sleep [2]. One was on sleep duration and sleep quality before and during pregnancy [3]. The report showed that the younger the pregnant women were, the more sleep difficulties they experienced; also the younger pregnant women felt more sleep-deprived despite having had enough sleep time during pregnancy. The other published study was on mothers’ sleep and their infants’ birth weight [4]. Findings showed no association between the amount or quality of mothers’ sleep and the risk of small-for-gestational-age birth weights among their infants.

Sleep disorders such as sleep-disordered breathing, restless legs syndrome, and periodic limb movement disorders [5] are known to be prevalent during pregnancy. It is very difficult to monitor sleep with polysomnography or without center sleep testing to detect sleep-disordered breathing or periodic limb movement during sleep in large epidemiological settings.

Further sleep studies of women during pregnancy may be needed both in epidemiological and clinical settings.