Breastfeeding during the first six months is linked to better sleep at one year

A recent study published in the European Journal of Clinical Nutrition provides evidence that breastfeeding during the first six months of life is associated with better sleep duration for infants at one year of age. The research suggests that babies who receive breast milk, either exclusively or in combination with formula, are less likely to experience abnormally short sleep periods compared to those who are only fed formula.

Adequate sleep is an essential component of a child’s healthy physical and psychological development. When infants consistently experience short sleep durations, they tend to face higher risks of future issues like obesity, hyperactivity, and behavioral challenges. These negative outcomes can adversely affect a child’s social skills and cognitive performance as they grow older.

The World Health Organization recommends exclusive breastfeeding for the first six months of life because of its proven benefits, including protection against childhood infections and support for healthy long-term development. Despite these widely recognized advantages, some caregivers choose infant formula based on the perception that breast milk is digested too quickly, leading to more frequent feedings and less overall sleep. Since infant sleep patterns change rapidly during the first year of life, scientists are looking for the early factors that shape these daily habits to help parents make informed feeding choices.

A research team led by Yuri Nakagawa, a doctoral researcher at the University of Toyama in Japan, sought to explore this potential connection. Nakagawa and her colleagues analyzed data from the Japan Environment and Children’s Study, which is one of the largest birth cohort projects in the world. The scientists designed their project to see if different feeding patterns in the first six months of life would predict how long infants slept at twelve months of age.

“WHO widely promotes breastfeeding, and most people are aware of the multiple health benefits it provides,” said Nakagawa, the study’s first author. “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.”

To gather their data, the researchers utilized information from a nationwide project that tracked pregnant women living across fifteen different regions in Japan. Participants originally enrolled in the study during their first trimester of pregnancy between January 2011 and March 2014. After excluding multiple births, miscarriages, stillbirths, and records with incomplete responses, the research team analyzed a final sample of 82,918 mother-infant pairs.

To measure feeding habits, mothers completed a self-administered questionnaire six months after giving birth. The mothers were asked to mark the duration of both breastfeeding and formula feeding by drawing lines in boxes that represented one-month intervals. Based on these responses, the researchers divided the infants into four distinct groups.

The first group consisted of babies who were exclusively formula-fed for the first six months of life. The second group included infants who were breastfed for less than six months. A third group was made up of babies who received a combination of breast milk and formula for a full six months. The final group consisted of infants who were exclusively breastfed for a full six months.

To assess the main outcome of the study, parents completed another questionnaire when their child reached one year of age. The parents documented their infant’s sleep schedule by marking thirty-minute intervals over a twenty-four-hour period, from midnight to midnight. The researchers then calculated the total daily sleep time for each child in the study.

The National Sleep Foundation in the United States recommends that one-year-old children sleep between eleven and fourteen hours every day. Relying on this pediatric guideline, the research team defined a short sleep duration as anything less than eleven hours of total sleep in a twenty-four-hour period. In their statistical models, the authors accounted for a wide range of potential confounding factors.

Confounding factors are outside variables that could independently affect both a mother’s feeding choices and her baby’s sleep duration. By controlling for these variables, the researchers could isolate the specific mathematical relationship between milk types and sleep habits. The analysis adjusted for maternal age, educational background, household income, smoking habits, postpartum depression, and the infant’s birth weight.

The team also accounted for the mother’s exercise habits during pregnancy, her level of social support, and any history of physical or mental illness. On the infant’s side, the researchers factored in the baby’s sleep duration at one month of age, the baby’s sex, daycare attendance, and even where the baby slept at night, such as in the parent’s bed or a separate crib.

The analysis revealed that breastfeeding during the first six months was consistently linked to a decreased risk of short sleep duration at one year of age. Among the infants who were exclusively formula-fed for six months, 12.2 percent experienced short sleep durations. This represented the highest rate of short sleep among all the infant groups in the study.

In comparison, the prevalence of short sleep was 10.2 percent for infants breastfed for less than six months. It was 9.7 percent for infants breastfed for six months alongside formula. The lowest rate of short sleep was observed in the exclusively breastfed group, sitting at just 8.8 percent.

When adjusting for the various confounding variables, the statistical differences between the groups remained significant. Compared to the exclusively formula-fed infants, the babies who were breastfed for less than six months had a 16 percent lower likelihood of short sleep. Infants who received a mix of breast milk and formula for six months had a 21 percent lower likelihood.

The strongest statistical association was seen in the exclusively breastfed group. These infants experienced a 23 percent lower likelihood of experiencing a short sleep duration at one year of age when compared to the babies who only received formula. The findings showed a graded association, meaning that longer breastfeeding durations were linked to a progressively lower risk of short sleep.

“This study provides reassurance against the common perception that breastfed babies sleep less because breast milk is digested more rapidly,” Nakagawa said. “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 biological mechanisms that might explain how breast milk promotes longer sleep in toddlers. One major difference between breast milk and infant formula is the presence of a hormone called melatonin. Melatonin helps regulate the baby’s internal clock and promotes the onset of sleep while improving overall sleep quality.

Newborn babies are largely unable to produce their own melatonin because their pineal gland, a tiny structure in the brain, is still developing. Because infant formula does not contain this hormone, breastfed infants receive a direct supply of maternal melatonin that formula-fed infants miss out on. This melatonin is secreted heavily into breast milk at night, which might help support the development of healthy sleep rhythms earlier in life.

Another key difference involves an essential amino acid called tryptophan, which the human body uses to produce melatonin. Because humans cannot synthesize tryptophan on their own, infants must acquire it entirely through their daily diet. While the nutritional composition of infant formula remains relatively constant, breast milk adapts to the changing needs of the infant throughout the day.

The concentration of tryptophan in breast milk fluctuates naturally, rising heavily during the nighttime hours. Infant formula maintains a constant level of tryptophan regardless of the time of day. The authors suggest that the natural nighttime increase of tryptophan in breast milk helps infants establish proper sleep and wake cycles.

Finally, the researchers pointed to the gut-brain axis as a possible explanation for the differences in sleep. The gut-brain axis is a complex communication network linking a person’s intestinal bacteria directly to their brain function. Breastfeeding heavily influences the composition of an infant’s gut microbiome, actively promoting the development of healthy bacteria.

Differences in this microbiota between breastfed and formula-fed infants may contribute to the development of healthy sleeping patterns. Since the gut microbes communicate directly with the brain through chemical signals, a healthier gut environment might actively promote better neurological development and improved sleep quality.

While the study provides evidence of a link between feeding methods and sleep, there are several limitations to consider. First, the researchers relied on self-reported questionnaires to gather data on both feeding habits and sleep schedules. This method introduces the possibility of recall bias, where parents might accidentally misremember or inaccurately report their baby’s routine.

The researchers also noted that the overall effect size was relatively modest. The maximum difference in short sleep prevalence between the exclusively breastfed group and the exclusively formula-fed group was only 3.4 percentage points. Parents who need to use formula should not misinterpret the findings to mean their child will inevitably suffer from severe sleep deprivation.

Another limitation is that the team did not directly measure the hormone levels or gut bacteria of the infants. The explanations regarding melatonin, tryptophan, and the gut microbiome are based on existing biological knowledge rather than direct medical tests from this specific group of babies. There may have also been unmeasured environmental factors that influenced the infants’ sleep habits.

For example, the study did not track the brightness of the room where the baby slept or the specific bedtime routines parents used to soothe their children. These external elements could play a major role in shaping a toddler’s resting patterns. Finally, the timeframes within the feeding categories varied widely across the thousands of participants.

For instance, the group of infants who were breastfed for less than six months could include babies who received breast milk for five months as well as babies who only breastfed for a single week. This variance makes it difficult to pinpoint the exact dosage of breast milk required to see a noticeable benefit. Future research should attempt to address these gaps by directly measuring biological markers like hormone levels and gut flora in breastfed and formula-fed infants.

The study, “Breastfeeding and children’s sleep duration at 1 year of age: A nationwide birth cohort – The Japan Environment and Children’s Study,” was authored by Yuri Nakagawa, Kenta Matsumura, Akiko Tsuchida, and Hidekuni Inadera.