A Japanese study observed better sleep quality in children fed breast milk, while a Norwegian study found an association between breastfeeding and a lower risk of ADHD.
Japanese researchers observed that children who received breast milk during the first six months of life had better sleep at one year of age compared to those fed exclusively formula. Adequate sleep in infancy is considered important for healthy physical and psychological development. "The WHO strongly promotes breastfeeding and most people are aware of its multiple health benefits," explained Yuri Nakagawa, the study's lead author. "Nevertheless, misconceptions persist, suggesting that breastfed infants sleep less or that formula-fed infants sleep longer. We wanted to provide concrete evidence to debunk this myth."
The study, published in the European Journal of Clinical Nutrition, analysed data from more than 82,918 mother-child pairs. When the children were six months old, mothers were given questionnaires about the feeding practices followed during their children's first six months of life. The children were then divided into four groups based on feeding method. The first group consisted of infants fed exclusively with formula. The second group included infants breastfed for less than six months. The third group included infants breastfed for all six months, with formula supplementation. The fourth group consisted of infants exclusively breastfed for six months. Upon reaching one year of age, parents completed another questionnaire indicating the child's sleep duration.
All groups of breastfed children showed a lower likelihood of insufficient sleep quality compared to infants fed exclusively formula. While infants fed only formula for the first six months had a 12.2 per cent probability of insufficient sleep, the risk in infants breastfed for less than six months was only 10.2 per cent. When breastfed for all six months and supplemented with formula, the risk was further reduced to 9.7 per cent. The lowest risk of insufficient sleep quality at one year of age was found in children exclusively breastfed for the first six months, at 8.8 per cent. After adjusting the data for a wide range of maternal, infant and environmental factors, infants exclusively breastfed for six months were 23 per cent less likely to have short sleep duration compared to those fed only formula.
The results also showed a gradual association, with longer breastfeeding duration linked to a progressively lower likelihood of insufficient sleep quality.
"This study debunks the widespread belief that breastfed infants sleep less because breast milk is digested more quickly," said Nakagawa. "Our findings suggest that such concerns should not discourage parents from considering breastfeeding and its many well-documented benefits."
The researchers proposed several possible explanations for the association between breast milk and sleep quality. While the nutritional composition of formula remains relatively constant, that of breast milk adapts to the changing needs of the newborn. To help establish and regulate the baby's biological clock and sleep-wake cycle, melatonin — a hormone that promotes sleep onset and improves sleep quality — is secreted into breast milk during the night. Since newborns produce only small amounts of melatonin, melatonin derived from breast milk may help support the development of a healthy sleep-wake rhythm.
Furthermore, breast milk contains tryptophan, an amino acid used in the production of melatonin. And tryptophan concentrations in breast milk are higher during the night. Additionally, it is hypothesised that differences in the gut microbiota between breastfed infants and formula-fed infants may also contribute to the development of healthy sleep-wake rhythms and better sleep quality.
Norwegian researchers, meanwhile, observed how breastfeeding up to six months of age is associated with a reduced risk of ADHD symptoms between the ages of three and eight. The study, published in the journal Biological Psychiatry, used data from 37,600 families. Mothers reported, via a questionnaire completed six months after birth, the duration of exclusive breastfeeding, partial breastfeeding and the introduction of other liquids or solid foods. These data were used to calculate the number of months each child was exclusively breastfed.
"We found that the longer a child was exclusively breastfed (up to six months), the lower the level of ADHD symptoms at three, five and eight years of age," said Berit Skretting Solberg, one of the study's authors. The association was observed in both males and females and was stronger at three and five years, and slightly weaker at eight years. All types of breastfeeding showed an effect, but the effect increased with the duration and intensity of breastfeeding and was strongest with exclusive breastfeeding up to six months.
The study took into account the known genetic risk of ADHD and sociodemographic factors. Sibling analyses were also conducted, comparing different breastfeeding patterns within the same family. "Even after these adjustments, a clear, albeit moderate, protective effect of exclusive breastfeeding duration on subsequent ADHD symptoms emerged," explained Solberg.
As with other observational studies, it is difficult to draw definitive conclusions about causality, so further research would be needed. As the researchers explained, heritability is probably the most important risk factor for ADHD. However, since ADHD, like other neurodevelopmental disorders, is influenced by multiple factors, the study suggests that the frequency of breastfeeding could also contribute to protecting against the development of ADHD symptoms in young children.