Influence of mothers’ nighttime responses on the sleep–wake rhythm of 1-month-old infants

The purpose of this study was to analyze the influence of the mothers’ nighttime responses on the sleep–wake rhythm of their 1-month-old infants. This study used an anonymous self-administered survey questionnaire with 1133 mothers of 1-month-old infants. The questionnaire investigated basic information about the parents, growth environment of infants, mothers’ sleep patterns during pregnancy, and infants’ sleep patterns at the age of one month. Logistic regression analysis was used to analyze the influence of nighttime responses on the risk of infants sleeping longer during the day than at night. Regarding nighttime response behavior, it was found that immediately picking up 1-month-old infants results in longer sleep during the day than at night (OR 1.616 [1.017 − 2.566], p = 0.042), compared to delaying picking up the infant. It was suggested that the stimulation due to picking up an infant may affect sleep–wake rhythm formation.

www.nature.com/scientificreports/ movements. However, intervening with a child's active sleep in this manner may inhibit the induction of quiet sleep 21 , which may increase daytime sleep and affect the formation and development of the sleep-wake rhythm. Furthermore, studies that investigated the relationship between nighttime responses to infants and sleep-wake rhythm formation reported that delaying picking up a baby or breastfeeding when they wake up during the night-soon after birth and five weeks after birth-extended nighttime sleep and reduced the frequency of nighttime waking patterns three months after birth 22 . While the previous study investigated three-month-old infants, much is yet to be elucidated about the influence at one month of age, which is before the sleep-wake rhythm is established 22 .
Therefore, we believe that by examining the impact of maternal intervention on the formation of the sleep-wake rhythm of 1-month-old infants during the night and conveying to mothers how to promote sleep-wake rhythm formation in their infants, it would be possible to improve the mental and physical health of both mothers and infants.

Methods
Ethics statement. This study was conducted with permission from the Kyushu University and Kurume University Ethics Committee. All experimental protocols were designed according to the Declaration of Helsinki's ethical principles and performed in accordance with the Ethical Guidelines for Medical and Health Research Involving Human Subjects. The participants provided informed consent after being given explanations about the purpose of the study, the fact that their participation was voluntary, and that withdrawal from the study midway would have no adverse consequences. Using a written document, the participants were informed that their personal information would be handled carefully and used only for the purposes of the study, that they may withdraw their consent at any time before the end of the study, and approximately how much time it would take to complete the experiments and the questionnaires.
Study population. Five institutions in Fukuoka Prefecture (two tertiary perinatal centers and three private maternity clinics) participated in this study. Mothers who came for a one-month health checkup with their infants between December 2014 and September 2018 were given questionnaires. The infants were healthy, fullterm, and one month old. Pre-term and post-term infants were excluded.
The questionnaires asked about basic information (e.g., age of mother and father, mother's history of childbirth, education history, household income, child's growth environment (e.g., breastfeeding method, lights-off time, brightness of the room(s) during the day), mother's sleep schedule during pregnancy (bedtime, period of sleep at night, and regularity of sleep one month before childbirth), and the child's sleep status at one month after birth (period of daytime sleep and period of nighttime sleep).
Under the growth environment of the child category, the mothers were asked to select from "Breastfeed right away, " "Pick up right away, " "Observe for a while, " and "Other" as responses to the question on "Nighttime intervention for the child. " "Daytime sleep" was the total number of hours slept between 8:00 AM and 8:00 PM, and "Nighttime sleep" was the total number of hours slept between 8:00 PM and 8:00 AM at one month of age. These times were set based on the annual sunrise/sunset times and the general social activity times in the study area in Japan.
Data and statistical analyses. We excluded cases with missing information about the daytime and nighttime sleep periods of the infant, as this was necessary for analysis. If daytime sleep was longer than nighttime sleep at one month of age, it was assumed that the formation of the sleep-wake rhythm had been delayed 14 . The children who slept more during the day were classified into the "Day sleep" group, while children who slept more during the night or slept equally during the night and day were classified into the "Night sleep" group. The basic characteristics of the "Day sleep" group and the "Night sleep" group were compared using the Mann-Whitney U test for quantitative variables and χ2 test for qualitative variables. We then carried out logistic regression analysis to understand the impact of nighttime interventions on the daytime sleep of 1-month-old infants in the "Day sleep" group. As covariates, we used factors considered to impact the formation of a child's sleep-wake rhythm (mother's age, mother's history of childbirth, gestational age, season during which the child was born, breastfeeding method, lights-off time, and mother's nighttime sleep during pregnancy). The analysis was carried out using the statistical software IBM SPSS version 25, with a significance level of 5%.

Results
Of the 1,420 cases who responded to the questionnaires, we excluded the responses of 14 cases of premature birth and 273 cases with missing information. Subsequently, data were analyzed for the remaining 1,133 pairs of infants and mothers. Table 1 shows the basic characteristics of the study population. The mean age of mothers was 32.0 ± 4.8 years and that of fathers was 33.5 ± 5.9 years. Regarding the mothers' history of childbirth, most of the participants were multipara mothers (n = 619; 54.6%). Majority of participants were college or university graduates (n = 829; 73.2%). In terms of household income, most participants stated that they were "Not comfortable, but satisfied" (n = 841; 74.2%). The family composition was such that most participants were part of a "nuclear family" (n = 926; 81.7%). The mean gestational age of the infants at birth was 39.5 ± 1.1 weeks, and the mean birth weight was 3.078 ± 379 g. The mean age of infants was 33.5 ± 5.9 days. Comparison between day sleep group and night sleep group. Table 2 shows the characteristics of the "Day sleep" group and the "Night sleep" group. There were significant differences in history of childbirth, season of birth of the infant, feeding method, lights-off time, and nighttime sleep of the mother during pregnancy.

Impact of nighttime intervention on the sleep of 1-month-old infants.
A logistic regression analysis was performed on the impact of mothers' nighttime responses on the daytime sleep of the infants of the "Day sleep" group (Table 3). Compared to those who responded with, "Observe for a while, " the risk of daytime sleep was significantly higher in those who chose the "Pick up right away" option (OR 1.616 [1.017-2.566]).

Discussion
We found that the nighttime response of immediately picking up a 1-month-old infant led to increased daytime sleep in those infants.
Effect of nighttime care on 1-month-old infants' sleep. When mothers thought their 1-month-old infants had woken up during the night, picking up infants immediately instead of simply observing them was associated with a greater risk of prolonged daytime sleep. We believe this is because the stimulation of being picked up inhibits active sleep among infants.  www.nature.com/scientificreports/ www.nature.com/scientificreports/ It is surmised that in most situations, the mother becomes aware that the child has woken up during the night when the infant is crying. However, after birth, mothers are sensitive to the movements of their infants during the night and are likely to wake up in response to these movements, even in the absence of crying 23 . Therefore, mothers can misinterpret such movements as their child having been awakened, even when they are not crying excessively, are not completely awake, or during active sleep. This can be explained by the fact that in the early stages after birth, infants in active sleep tend to exhibit movements that make them appear as if they are awake 15 . When examining the flow from sleep to awakening, not all stimuli during sleep lead to waking up. Depending on the extent of the stimulus, infants either wake up completely or return to sleep 24 . Therefore, a mother may assume that her child has woken up even though the child is still in active sleep. The stimulation from being picked up may cause the child to wake up and, thus, inhibit the child's active sleep. This may in turn inhibit the induction of quiet sleep 21 and reduce nighttime sleep. In addition, the active sleep of infants immediately after birth develops the nerves in the suprachiasmatic nucleus and hypothalamus of the brain that control the circadian/sleep-wake rhythm 25 . These studies suggest that inhibition of active sleep may influence the formation of the sleep-wake rhythm. Active sleep in infants, particularly in the early stages after birth, is important for brain development. Therefore, if parents or caregivers believe that their child has woken up during the night, unless there are clear signs of crying, it is better to observe infants for a while before picking them up, rather than picking them up immediately. It is particularly difficult to distinguish between complete awakening and active sleep in the beginning; therefore, using the aforementioned approach can prevent the infant's active sleep from being inhibited.
James-Roberts et al. studied the link between nighttime responses for infants at birth and five weeks after birth and the quality of sleep in infants at 3 months after birth, and reported that the duration of nighttime sleep at three months of age was longer in infants whose mothers delayed picking up their babies when they had woken up during the night, and indicated that observing and taking time to pick up their child during the night worked positively toward the child's sleep-wake rhythm 22 . Taking time to respond to an infant at night may also help increase the infant's self-calming power. It has been reported that the active/quiet sleep cycle in infants is approximately 45 to 60 min/cycle, and physiological signs of awakening manifest between sleep cycles 9 . If the infant is not allowed to sleep after physiological awakening and transition to the next sleep cycle, sleep can be disrupted. In contrast, infants who are allowed the transition naturally to the next sleep cycle learn how to calm themselves and are able to go back to sleep on their own and reinforce their sleep. Considering this, we believe that observing and refraining from immediately picking up the infant when he or she awakes during the night helps improve the infant's self-calming power and promotes the formation of a proper sleep-wake rhythm.
Effect of breastfeeding on 1-month-old infants' sleep. If the mother immediately picks up an awakening child during the night, it can delay the formation of a proper sleep-wake rhythm in the infant; however, there was no relationship between "breastfeeding right away" and the formation of the sleep-wake rhythm. We believe this is because of the difference in the extent of stimulus between picking up and breastfeeding, which leads to the child waking up. It is difficult to define what "awake" is in infants with immature sleep development 24 . Lijowska et al. defined "awake" as a series of events from sighs, startles, and thrashing limb movements, to full arousal 26 . Furthermore, not all stimuli during sleep lead to the infant waking up. Depending on the extent and intensity of the stimulus, infants may either wake up entirely or return to sleep on their own. In this study, we understood that picking up the child, which involves lifting up and swinging the child, is a strong stimulus that leads to complete awakening, in contrast to breastfeeding, which is a low-intensity stimulus that does not lead to complete awakening. The reason we felt that breastfeeding had no impact on the formation of an infant's sleep-wake rhythm was the possible influence of melatonin in breastmilk. Melatonin is a hypnotic hormone that is produced in the mother's body and transferred to the breastmilk 27,28 . Therefore, we believe that the hypnotic effect of melatonin acts on infants through breastmilk and ensures nighttime sleep time.
Study strength and limitations. This study derives it strength from its novelty as it is the first to show the influence of mothers' nighttime responses on the sleep-wake rhythm of 1-month-old infants. The data used in the study were derived from several clinic sites and included primiparas and multiparas while controlling for parity in the analysis. However, this study had a few limitations. This was an observational study, so there could be confounding factors that were not measured and accounted for. Moreover, the participants were evaluated using a self-reported questionnaire, so there could be some bias. Although the number of people who refused to answer the questionnaire was extremely low, the exact number and background of those who refused could not be ascertained. Further, there was insufficient research on the effects of maternal circadian rhythms other than the mothers' nighttime sleep during pregnancy. Since the questionnaire was answered by the mother, it was assumed that the mother took care of the child at night, but no clear distinction was made as to whether the mother or the father took care of the child. As this study was limited to low-risk mothers and infants with full-term delivery without any obvious complications, we did not examine the impact on high-risk mothers and infants. The investigation was based on responses given to questionnaires; thus, the data we obtained were lacking in detail compared to using sleep charts and actigraphy for gathering sleep data on mothers and infants. While the act of picking up the infant was thought to affect the sleep-wake rhythm by stimulating infant awakening, we cannot confirm the precise nature of the act, since we did not study the process of the infants being picked up. Finally, as this study was conducted in Japan, its generalizability beyond the Japanese context is limited.