Breast milk, best sleep: Nationwide Japanese study finds

The World Health Organization (WHO) recommends exclusive breastfeeding for the first six months of life because of its many proven benefits, including protection against infections and support for healthy long-term development. However, perceptions that breastfed infants sleep less or require more frequent feeding than formula-fed infants remain common among parents and caregivers. Short sleep duration during infancy has also been linked to obesity, behavioral problems, and poorer cognitive performance later in life. Therefore, adequate sleep during this period is considered important for healthy physical and psychological development. Although infants are known to gradually develop longer and consolidated sleep periods, some caregivers choose formula feeding partly based on concerns about their child’s sleep.

To better understand the relationship between infant feeding and sleep, Ms. Yuri Nakagawa, a doctoral researcher at the University of Toyama, Japan, and colleagues analyzed data from the nationwide Japan Environment and Children's Study (JECS), one of the world's largest birth cohort studies. The study examined 82,918 mother–infant pairs to investigate whether feeding practices during the first six months of life were associated with sleep duration at one year of age. The findings were published in the European Journal of Clinical Nutrition on March 31, 2026.

“WHO widely promotes breastfeeding, and most people are aware of the multiple health benefits it provides. Nevertheless, perceptions that breastfed infants sleep less, or that formula-fed infants sleep for longer periods, remain common. We wanted to provide solid evidence to bust this misconception,” says Ms. Nakagawa, the study's first author.

The mothers were given questionnaires at 6 months asking about the feeding practices they followed during the first six months for their babies. The children were then divided into four groups according to their feeding method. The first group consisted of infants fed exclusively with formula. The second group included infants who were breastfed for less than six months. The third group included infants who were breastfed throughout the six-month period while also receiving formula supplementation. The fourth group consisted of infants who were exclusively breastfed for six months. When the children reached one year of age, parents completed another questionnaire reporting their child's sleep duration. Children sleeping less than 11 hours per day were considered to have insufficient sleep, based on the U.S. National Sleep Foundation recommendations.

All groups that received breastmilk showed lesser chance of insufficient sleep compared to exclusively formula-fed infants. While infants who received only formula for the first six months had a 12.2% chance of having short sleep, the risk in infants breastfed for less than 6 months was only 10.2%. When breastfed for the entire six months and supplemented with formula, the risk further fell to 9.7%. The least risk of insufficient sleep at age one was for babies exclusively breastfed for the first six months, at 8.8%. After adjusting for a wide range of maternal, infant, and environmental factors, infants who were exclusively breastfed for six months had a 23% lower likelihood of short sleep duration compared with those fed only formula. The findings also showed a graded association, with longer breastfeeding duration associated with a progressively lower likelihood of short sleep.

“This study provides reassurance against the common perception that breastfed babies sleep less because breast milk is digested more rapidly,” says Ms. Nakagawa. “Our findings suggest that such concerns should not discourage parents from considering breastfeeding and its many well-established benefits,” she adds.

The researchers proposed several possible explanations. While the nutritional composition of formula remains relatively constant, that of breast milk adapts to the changing needs of the infant. To help establish and regulate the baby's internal clock and sleep–wake cycle, melatonin—a hormone that promotes sleep onset and improves sleep quality—is secreted into breast milk at night. Because newborns produce only small amounts of their own melatonin, breast milk-derived melatonin may help support the development of healthy sleep rhythms. In addition, breast milk contains tryptophan, an amino acid used to produce melatonin. Interestingly, tryptophan concentrations in breast milk have also been found to be higher at night.

Furthermore, growing evidence supports the gut–brain axis, a communication network linking intestinal bacteria and brain function. Breastfeeding is known to positively influence the development of a healthy infant gut microbiome. Differences in gut microbiota between breastfed and formula-fed infants may also contribute to the development of healthy sleep–wake patterns and sleep quality.

Reference:

Nakagawa, Y., Matsumura, K., Tsuchida, A. et al. Breastfeeding and children’s sleep duration at 1 year of age: A nationwide birth cohort - The Japan Environment and Children’s Study. Eur J Clin Nutr 80, 476–482 (2026). https://doi.org/10.1038/s41430-026-01718-1