The purpose of the present study is to examine the association between toddlers' sleep arrangements and their nighttime sleep duration and other sleep variables. For this investigation, we performed a study in which child activity and sleep levels were recorded using actigraphy. The parents of 1.5-year-old toddlers (n = 106) were asked to attach an actigraphy unit to their child’s waist with an adjustable elastic belt and complete a sleep diary for 7 consecutive days. Questionnaires were used to assess the sleep arrangements of the toddlers. There was a significant negative correlation between nap duration and nighttime sleep duration, suggesting that longer nap sleep induces shorter nighttime sleep duration. Among the sleep arrangements, such as nighttime breastfeeding or co-sleeping, only nighttime breastfeeding predicted shorter nighttime sleep duration. Our findings indicate that shorter naps induce a longer nighttime sleep in 1.5-year-old toddlers while nighttime breastfeeding decreases their nighttime sleep duration.

Previous studies have demonstrated that afternoon naps can have a negative effect on subsequent nighttime sleep in children. These studies have mainly been based on sleep questionnaires completed by parents. To investigate the effect of napping on such aspects of sleep quality, we performed a study in which child activity and sleep levels were recorded using actigraphy. The parents were asked to attach actigraphy units to their child's waist by an adjustable elastic belt and complete a sleep diary for 7 consecutive days. 50 healthy young toddlers of approximately 1.5 years of age were recruited. There was a significant negative correlation between nap duration and both nighttime sleep duration and sleep onset time, suggesting that long nap sleep induces short nighttime sleep duration and late sleep onset time. We also found a significant negative correlation between nap timing and nighttime sleep duration and also a significant positive correlation between nap timing and sleep onset time, suggesting that naps in the late afternoon also lead to short nighttime sleep duration and late sleep onset. Our findings suggest that duration-controlled naps starting early in the afternoon can induce a longer nighttime sleep in full-term infants of approximately 1.5 years of age.

Using uterine explants from Per1::Luc rats and in situ hybridization, we recently reported that the circadian property of the molecular clock in the uterus and placenta is stably maintained from non-pregnancy, right through to the end stage of pregnancy under regular light-dark (LD) cycles. Despite long-lasting increases in progesterone during gestation and an increase in estrogen before delivery, the uterus keeps a stable Per1::Luc rhythm throughout the pregnancy. The study suggests the importance of stable circadian environments for fetuses to achieve sound physiology and intrauterine development. This idea is also supported by epidemiological and animal studies, in which pregnant females exposed to repeated shifting of the LD cycles have increased rates of reproductive abnormalities and adverse pregnancy outcomes. Leading from this, we introduced artificial circadian environments with controlled lighting conditions to human preterm infants by developing and utilizing a specific light filter which takes advantage of the unique characteristics of infants' developing visual photoreceptors. In spite of growing evidence of the physiological benefits of nighttime exposure to darkness for infant development, many Japanese Neonatal Intensive Care Units (NICUs) still prefer to maintain constant light in preparation for any possible emergencies concerning infants in incubators. To protect infants from the negative effects of constant light on their development in the NICU, we have developed a new device similar to a magic mirror, by which preterm infants can be shielded from exposure to their visible wavelengths of light even in the constant light conditions of the NICU while simultaneously allowing medical care staff to visually monitor preterm infants adequately. The device leads to significantly increased infant activity during daytime than during night time and better weight gains.