A large-scale study from Japan has found that infants who are breastfed, particularly those exclusively breastfed for the first six months, tend to sleep longer at one year of age than those fed only formula. The findings, published in the European Journal of Clinical Nutrition, challenge a common perception that breastfed babies sleep less because breast milk is digested more quickly.
Researchers from the University of Toyama analysed data from 82,918 mother-infant pairs enrolled in the nationwide Japan Environment and Children's Study. They categorised infants into four groups: those fed exclusively with formula; those breastfed for less than six months; those breastfed for six months with formula supplements; and those exclusively breastfed for the full six months. Mothers reported feeding practices when their babies were six months old, and parents completed a second questionnaire on sleep duration when the children turned one.
The results showed that all groups who received breast milk had lower odds of short sleep — defined as less than 11 hours in a 24-hour period, based on recommendations from the US-based National Sleep Foundation — compared with infants fed only formula. The probability of short sleep was 12.2% in the formula-only group, falling to 10.2% among those breastfed for less than six months, 9.7% for those breastfed with formula supplements, and 8.8% for those exclusively breastfed. After adjusting for maternal, infant, and environmental factors, exclusive breastfeeding for six months was associated with a 23% lower likelihood of short sleep duration.
Biological Mechanisms Behind the Link
The study’s lead author, Yuri Nakagawa, said the research aimed to provide solid evidence to counter the misconception that breastfed infants sleep less. “Perceptions that breastfed infants sleep less, or that formula-fed infants sleep for longer periods, remain common,” she said in a press release. “We wanted to provide solid evidence to bust this misconception.”
The researchers proposed several biological explanations. Unlike formula, whose nutritional composition remains fixed, breast milk adapts to an infant’s changing needs. Notably, melatonin — a hormone that helps initiate sleep and regulate the sleep-wake cycle — is secreted into breast milk at night. Newborns produce only small amounts of their own melatonin, so receiving it through breast milk may support the development of healthy sleep patterns. Breast milk also contains tryptophan, an amino acid the body uses to produce melatonin, and its concentrations rise at night.
Growing evidence also points to the role of the gut-brain axis, a communication network linking gut bacteria and brain function. Breastfeeding positively influences the development of a healthy infant gut microbiome, and differences in gut microbiota between breastfed and formula-fed babies may contribute to healthier sleep-wake patterns and better sleep quality.
The World Health Organization recommends exclusive breastfeeding for the first six months of life due to its proven benefits, including protection against infection and support for healthy long-term growth. Nakagawa stressed that concerns about sleep should not discourage parents from considering breastfeeding. “WHO widely promotes breastfeeding, and most people are aware of the multiple health benefits it provides,” she said.
While the study is based on Japanese data, its findings align with broader research on infant nutrition and sleep. A separate Norwegian study has linked exclusive breastfeeding to lower ADHD symptoms in children, further underscoring the potential long-term benefits of breastfeeding for neurodevelopment.
