Background and objectives: Recently, Japanese infant's bedtime has become late and it is reported that this might have a bad influence on infant's growth. The purpose of the present study was to investigate the actual situation and interrelationships between night waking and other sleep problems in Japanese 4-month -old infant.
Methods: The subjects were 194 mothers and infants who participated in health checkups at four months after delivery in Fukuoka City. The questionnaire consisted of 3 components: 1) infant's and mother's sleep practices and sleep problems; 2) coping behavior for infant's sleep; 3) perceptions of child-care and maternal health. Their responses were linked to health checkup's results. The subjects were divided into two groups; Waking Group (n = 111) who was wakening one or more times from 0-6 a.m. and Sleeping Group (n = 83) who was sleeping throughout the night. The proportion of Waking Group was constituted 57.2% in all infants. Infant's growth, sleep, and sleep problems were compared between two groups.
Results: The mean infant's bedtime was 10:28 p.m., and the proportion of infants reported to go to bed at 10 p.m. or later was 69.4%. The proportion of infants with irregular bedtimes was 16.5%. Twenty-eight point six percent of infants had sleep problems like difficulty settling, severe night waking and were suspected to be high risk of sleep disorders. Infants in the Waking Group were found to have more numbers of sleep problems than in the Sleeping Group. The proportion with difficulty settling was higher in the Waking Group. Additionally, the proportion with irregular bedtimes was higher in the Waking Group. However, infant's height and weight did not significantly differ between two groups.
Conclusions: Late bedtime and high proportion of night waking in 4-month-old infants were found to be characteristic in Fukuoka city. There is a possibility that night waking reflects delayed development of circadian rhythms.