Recent research published in European Journal of Clinical Nutrition provide evidence that breastfeeding during the first 6 months of life is associated with improved sleep duration in infants at 1 year of age. This study suggests that babies who are fed exclusively breast milk, or in combination with formula, are less likely to experience abnormally short sleep duration compared to babies who are fed only formula.
Adequate sleep is an essential element for a child’s healthy physical and psychological development. If infants consistently get less sleep, they tend to be at increased risk for future problems such as obesity, hyperactivity, and behavioral challenges. These negative outcomes can negatively impact a child’s social skills and cognitive abilities as they grow older.
The World Health Organization recommends exclusive breastfeeding for the first six months of life due to the proven benefits of breastfeeding, including preventing childhood infections and supporting healthy long-term development. Despite widespread recognition of these benefits, some caregivers choose formula based on the perception that breast milk is digested too quickly, leading to more feedings and less overall sleep time. Because infants’ sleep patterns change rapidly during the first year of life, scientists are looking for early factors that shape these daily habits to help parents make informed feeding choices.
A research team led by Yuri Nakagawa, a postdoctoral fellow at the University of Toyama in Japan, sought to explore this potential link. Nakagawa and his colleagues analyzed data from the Japan Environment and Children Survey, one of the world’s largest birth cohort projects. Scientists designed a project to see if different feeding patterns at six months of age could predict infants’ sleep duration at 12 months.
“The WHO has widely promoted breastfeeding, and most people are aware of the various health benefits of breastfeeding,” said Nakagawa, lead author of the study. “Despite this, there is still a common perception that breastfed infants sleep less, or that formula-fed infants sleep longer. We wanted to provide hard evidence to dispel this misconception.”
To collect the data, the researchers used information from a nationwide project that followed pregnant women living in 15 different regions of Japan. Participants initially enrolled in the study during their first trimester from January 2011 to March 2014. After excluding records of multiple births, miscarriages, stillbirths, and incomplete responses, the researchers analyzed a final sample of 82,918 mother-child pairs.
To measure feeding habits, mothers completed a self-administered questionnaire 6 months after giving birth. Mothers were asked to mark the duration of both breastfeeding and formula feeding by drawing a line within a box representing the 1-month interval. Based on these responses, the researchers divided the infants into four different groups.
The first group consisted of babies who were exclusively fed milk for the first 6 months of life. The second group included infants younger than 6 months who had been breastfed. The third group consisted of babies who were fed a combination of breast milk and formula for a full six months. The final group consisted of infants who were exclusively breastfed for six full months.
To assess the main results of the study, parents completed another questionnaire when their children turned 1 year old. Parents recorded their infant’s sleep schedule by marking 30-minute intervals over a 24-hour period from midnight to midnight. The researchers then calculated the total number of hours of sleep per day for each child in the study.
The National Sleep Foundation recommends that 1-year-olds sleep between 11 and 14 hours each day. Based on this pediatric guideline, the research team defined short sleep duration as less than 11 hours of total sleep in a 24-hour period. The authors considered a wide range of potential confounders in their statistical models.
Confounders are external variables that can independently influence both the mother’s food choices and the baby’s sleep duration. By controlling for these variables, the researchers were able to isolate a specific mathematical relationship between milk type and sleep habits. Analyzes adjusted for maternal age, education, household income, smoking habits, postpartum depression, and infant birth weight.
The research team also considered the mothers’ exercise habits, level of social support, and history of physical or mental illness during pregnancy. On the infant side, researchers took into account the baby’s sleep duration during the first month of life, the baby’s gender, daycare attendance, and even where the baby slept at night, such as in the parent’s bed or in a separate crib.
Analysis revealed that breastfeeding during the first 6 months was consistently associated with a reduced risk of short sleep duration at 1 year of age. Among infants who were exclusively formula-fed for six months, 12.2 percent experienced reduced sleep duration. This represented the highest rate of short sleep of all groups of infants in the study.
In contrast, 10.2 percent of breastfed infants less than 6 months of age had short sleep duration. The rate was 9.7% for infants who were breastfed for 6 months in combination with formula. The lowest rate of sleep deprivation was in the exclusive breastfeeding group at just 8.8%.
After adjusting for various confounding variables, the statistical differences between groups remained significant. Compared to infants who were exclusively formula-fed, infants less than six months old who were breastfed were 16 percent less likely to have short sleep duration. Infants who were fed a mixture of breast milk and formula for six months were 21 percent less likely.
The strongest statistical association was found in the exclusively breastfed group. These infants were 23 percent less likely to experience reduced sleep duration at one year of age compared to infants who were fed only formula. The findings show a graded association, meaning that longer breastfeeding durations are associated with a progressively lower risk of sleep deprivation.
“This study provides reassurance to the common perception that breastfed babies sleep less because breast milk is digested more quickly,” Professor Nakagawa said. “Our findings suggest that such concerns should not prevent parents from considering breastfeeding and its many well-established benefits,” she added.
Researchers have proposed several biological mechanisms that may explain how breast milk promotes longer sleep in infants. One of the major differences between breast milk and infant formula is the presence of a hormone called melatonin. Melatonin regulates your baby’s internal clock and promotes sleep onset while improving overall sleep quality.
Newborn babies are barely able to produce their own melatonin because the pineal gland, a small structure in the brain, is still developing. Because infant formula does not contain this hormone, breastfed infants receive a direct source of maternal melatonin that formula-fed infants miss out on. This melatonin is secreted in large amounts into breast milk at night and may support the development of healthy sleep rhythms early in life.
Another important difference involves an essential amino acid called tryptophan that the human body uses to produce melatonin. Humans cannot synthesize tryptophan on their own, so infants must obtain tryptophan entirely from their daily diet. While the nutritional content of infant formula remains relatively constant, breast milk adapts to the infant’s changing needs throughout the day.
The concentration of tryptophan in breast milk fluctuates naturally and increases significantly during the night. Infant milk maintains a constant level of tryptophan regardless of the time of day. The authors suggest that tryptophan in breast milk increases naturally during the night, helping infants establish proper sleep-wake cycles.
Finally, the researchers pointed to the gut-brain axis as a possible explanation for the sleep differences. The gut-brain axis is a complex communication network that directly links human gut bacteria and brain function. Breastfeeding has a significant impact on the composition of an infant’s gut microbiome, actively promoting the development of healthy bacteria.
Differences in the microbiome between breastfed and formula-fed infants may contribute to the development of healthy sleep patterns. Because gut bacteria communicate directly with the brain through chemical signals, a healthier gut environment may actively promote improved neurological development and better sleep quality.
Although this study provides evidence of an association between feeding style and sleep, there are some limitations that should be considered. First, the researchers collected data on both eating habits and sleep schedules based on self-reported questionnaires. This method introduces the potential for recall bias, where parents may misremember or inaccurately report their baby’s daily routine.
The researchers also noted that the overall effect size was relatively modest. The maximum difference in short sleep rates between the exclusively breastfed and exclusively formula-fed groups was only 3.4 percentage points. Parents who must use formula should not mistake this finding to mean that their child will inevitably suffer from severe sleep deprivation.
Another limitation is that the researchers did not directly measure the infants’ hormone levels or gut bacteria. Descriptions of melatonin, tryptophan, and the gut microbiome are based on existing biological knowledge rather than direct medical testing from this particular group of infants. There may also have been unmeasured environmental factors influencing infant sleep habits.
For example, the study didn’t track things like the brightness of the room babies slept in or specific bedtime routines that parents used to soothe their children. These external factors may play a major role in shaping infants’ rest patterns. Finally, time frames within feeding categories varied widely among thousands of participants.
For example, a group of infants who were breastfed for less than 6 months might include infants who were breastfed for only 1 week, as well as infants who were breastfed for 5 months. This difference makes it difficult to pinpoint the exact dose of breast milk needed to achieve a noticeable effect. Future studies should attempt to address these gaps by directly measuring biological markers such as hormone levels and gut microbiota in breastfed and formula-fed infants.
The study, “Breastfeeding at 1 year of age and children’s sleep duration: National birth cohort – Japanese environment and child study,” was authored by Yuri Nakagawa, Kenta Matsumura, Akiko Tsuchida, and Hidekuni Inadera.
