Breastfeeding linked to longer sleep in babies' first year, study finds
Japanese researchers found that one-year-olds who were breastfed during their first six months slept for longer than those who were formula-fed. One possible explanation is that the nutrients in formula remain largely fixed, while breast milk adapts to a baby's changing nutritional needs.
Infants exclusively breastfed for their first six months hadlower odds of short sleep duration — defined as sleeping less than 11 hours in 24 hours at age one — than infants who were exclusively formula-fed, researchers found after analysing data from 82,918 mother-infant pairs in the nationwide Japan Environment and Children's Study.
Children who received a combination of breastfeeding and formula also had lower odds of short sleep, suggesting the association was not limited to exclusive breastfeeding.
"Perceptions that breastfed infants sleep less, or that formula-fed infants sleep for longer periods, remain common," Yuri Nakagawa from the University of Toyama, the study’s lead author, said in a press release. "We wanted to provide solid evidence to bust this misconception."
Nakagawa added that the study offers reassurance against the widespread belief that breastfed babies sleep less because breast milk is digested more quickly.
She stressed that such concerns should not discourage parents from considering breastfeeding and its many well-established benefits. "WHO widely promotes breastfeeding, and most people are aware of the multiple health benefits it provides," Nakagawa said.
The researchers divided the children into four groups: the first comprised infants fed exclusively with formula milk; the second those who were breastfed for less than six months; the third infants who continued breastfeeding for the full six months alongside formula supplements; and the fourth infants who were exclusively breastfed for the entire six months.
Mothers were given questionnaires when their babies were six months old to report the feeding practices they had followed.
When the children reached their first birthday, parents completed another questionnaire on how long their babies slept. The researchers drew on recommendations from the US-based National Sleep Foundation, which defines short sleep as less than 11 hours per day.
The likelihood of short sleep
The results, published in the European Journal of Clinical Nutrition, showed that all groups who received breast milk were less likely to experience sleep deprivation than peers who relied solely on formula.
The probability of short sleep in the group fed exclusively with formula was 12.2%, while it fell to 10.2% among those breastfed for less than six months.
For infants who continued breastfeeding throughout the six months with the addition of formula milk, the risk fell to 9.7%.
The lowest risk of all was recorded among babies who were exclusively breastfed during the first six months, at just 8.8%.
After the data were adjusted to account for maternal, infant and environmental factors, it emerged that infants who were exclusively breastfed for six months were 23% less likely to have short sleep duration compared with those fed only formula.
Longer breastfeeding duration was also associated with a progressively lower likelihood of short sleep.
Biological explanations behind the findings
The researchers put forward several possible explanations for this phenomenon. While the nutritional composition of formula milk remains relatively constant, the composition of breast milk adapts to the changing needs of the infant.
The team noted that melatonin, a hormone that helps initiate sleep and improves its quality, is secreted into breast milk at night to help establish and regulate the baby’s internal clock and sleep–wake cycle.
Because newborns produce only small amounts of their own melatonin, receiving this hormone 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 previous research has found that its concentrations in breast milk also rise at night.
Growing evidence points to the role of the "gut–brain axis", a communication network linking gut bacteria and brain function.
Breastfeeding has a positive impact on the development of a healthy infant gut microbiome, and differences in gut microbiota between breastfed babies and those fed formula may contribute to the development of healthier sleep–wake patterns and better sleep quality.
The World Health Organization recommends exclusive breastfeeding during the first six months of a baby’s life because of its many proven benefits, including protection against infection and support for healthy long-term growth.
Short sleep duration in infancy has been linked to obesity, behavioural problems and poorer cognitive performance later in life, making adequate sleep during this period essential for healthy physical and psychological development.
