Two University of Maine researchers working in conjunction with colleagues in Japan have found links between infant and child sleeping arrangements and the phenomenon of "yonaki," or nighttime crying.
The research suggests how parents, infants and toddlers all can get a better night's sleep by making bedtime arrangements as consistent as possible.
Marie Hayes, professor of psychology, and Michio Fukumizu, a pediatric neurologist from Tokyo, Japan, and visiting scholar in the UMaine psychology department, with two Japanese co-researchers have identified several factors in an infant's first few months of life that can affect how they sleep and how often they awaken in distress during the night.
The results of their research were published in an article, "Sleep-Related Nighttime Crying (Yonaki) in Japan: A Community-Based Study," in the January issue of the journal Pediatrics, with colleagues and co-authors Makiko Kaga, M.D., director of the National Institute of Mental Health, National Center of Neurology and Psychiatry in Japan, and Jun Kohyama, M.D., associate director of Tokyo Kita Shakai Hoken Hospital.
The study involved interviews with the parents of 500 infants, toddlers and children at a well-infant clinic in Tokyo. It is customary in Japan for parents to bring infants and toddlers into their sleeping quarters – typically a mattress on the floor (tatami) to nurse and fall asleep. Later, most infants stay with the parents, a practice termed "co-sleeping," which makes the child more accessible to the parent during the night. For a small percentage of infants, sleep occurs in a separate crib-like bed away from the parents but in the same room.
The study's findings suggest a critical factor contributing to night-waking is not as much co-sleeping or separate sleeping arrangements, but rather changes in where the infant sleeps during the night and during naps.
"In other words, it is consistency in sleeping arrangements from sleep onset until awakening that appears to be critical," Hayes says. "Infants and toddlers are disrupted when the sleeping arrangements, i.e. the site of sleep, are variable or changed between sleep onset and later in the sleep period."
In the research paper, Fukumizu and Hayes say, "We are arguing that neither co-sleeping nor separate sleeping quarters matters fundamentally. Rather, it is the consistency in sleep site from the transition to sleep throughout the sleep period is most important. Parents should avoid "musical" beds, particularly during early infancy when night-feedings are common. We want to tell people that whatever they decide on for a sleeping arrangement, it must be kept the same."
Changing sleeping arrangements is disruptive to sleep in infants and toddlers possibly because infants are sensitive to what a familiar environment, particularly during a long sleep period. An infant waking up in a different place than where it went to sleep, Hayes and Fukumizu suggest, may be disorienting and contribute to night-time anxiety.
The study also found that toddlers with frequent sleep-related night-time crying (SRNC) were more likely to have irregular bedtimes and to have non-parental day care than were those without sleep-related night crying. And preschoolers who typically slept 9.5 to 10.5 hours per night were found less likely to experience SRNC than children with longer or shorter sleep periods. In addition to preventing parents from getting a good night's sleep, children in all groups experiencing frequent SRNC were more likely to suffer from a chronic eczema skin condition, and toddlers and preschoolers with SRNC exhibited bruxism (grinding teeth) more frequently, the researches found. Both conditions have been associated with anxiety.
Fukumizu says the impetus for the research was to try to shed light on a growing problem in Japan with new parents, who are becoming fewer as Japan's birth rates decline at a rate that has concerned the nation. There has been little research done on sleep problems with infants and toddlers in Japan and almost none about "yonaki" written in English, he says.
"In the past, only the father worked in Japan, but now both parents working is very common," he says. "Yonaki or night-waking makes the parents – especially the mother – exhausted due to sleep deprivation. Sometimes child abuse occurs because of babies non-stop crying."
Pediatricians have insufficient knowledge about SRNC and medical schools in Japan also have little knowledge about sleep, developmental behavioral pediatrics and child psychiatry. It is a new field for research, Fukumizu says.
Many tired parents have been asking pediatricians why their children cry at night and, says Fukumizu, "I wanted to contribute to the comprehension of infant and toddler sleep problems."
The research findings have universal application to other cultures besides Japan because co-sleeping and particularly, partial co-sleeping are increasingly common in the U.S. For this reason, these results have general implications for recommendations to parents and for describing the development of consolidated, full-night sleeping in infants and toddlers.