• Posted September 5, 2025

Breast Milk Timing Crucial For Babies' Sleep, Researchers Say

Busy moms might be sending their babies the wrong signal if they feed evening breast milk that was expressed in the morning, a new study suggests.

The composition of breast milk changes throughout the day, including hormones thought to influence babies’ wake/sleep patterns, researchers reported today in Frontiers in Nutrition.

A mother might unintentionally disturb her infant’s rest if she stores breast milk in the morning and then provides it in the afternoon or evening, researchers warned.

“Breast milk is a dynamic food: Consideration should be given to the time it is fed to the infant when expressed breast milk is used,” lead researcher Melissa Woortman said in a news release. She’s a recent doctoral graduate in nutritional sciences of Rutgers University in New Brunswick, N.J.

“The timing of these cues would be particularly critical in early life, when the infant’s internal circadian clock is still maturing,” senior investigator Maria Gloria Dominguez-Bello, a professor in biochemistry and microbiology at Rutgers, said in a news release.

Doctors consider breast milk a baby “super food” that is chock full of vitamins, minerals and compounds that help build the young immune system and feed the growing body, researchers said.

It’s widely thought to be the best source of infant nutrition, but many moms aren’t able to directly breastfeed multiple times throughout the day and night. Instead, they use a pump to express and store milk for later feedings.

For the study, researchers took 10 milliliter breast milk samples from 21 women at specific intervals on two days, roughly a month apart: 6 a.m., noon, 6 p.m. and midnight. Another 17 participants provided one day’s worth of samples taken at the same times.

Researchers analyzed the samples for levels of three hormones: melatonin, cortisol and oxytocin. Melatonin and cortisol are involved in regulation of wake/sleep cycles.

The breast milk samples also were analyzed for levels of immunoglobulin A, an antibody protein produced by the immune system, and the milk protein lactoferrin. These influence the baby’s digestive system.

Results showed that melatonin and cortisol varied over the course of a day. Melatonin peaks at midnight, while cortisol is highest in the early morning.

“We all have circadian rhythms in our blood, and in lactating mothers, these are often reflected in breast milk,” Woortman said. “Hormones like melatonin and cortisol follow these rhythms and enter milk from maternal circulation.”

The other breast milk components were mostly stable throughout the day. This might be because they aren’t as strongly influenced by signals dictating circadian rhythms, researchers said.

The results suggest that moms should try to feed their expressed milk to reflect the time when it was gathered and stored, researchers said.

“Labeling expressed milk as ‘morning,’ ‘afternoon,’ or ‘evening’ and feeding it correspondingly could help align expressing and feeding times and preserve the natural hormonal and microbial composition of the milk, as well as circadian signals,” Dominguez-Bello said.

For busy moms, that adjustment stands to be a practical one, Woortman said.  

“In modern societies where it may not be feasible for mothers to stay with their infants throughout the day, aligning feeding times with the time of milk expression is a simple, practical step that maximizes the benefits of breast milk when feeding expressed milk,” she concluded.

More information

The U.S. Centers for Disease Control and Prevention has more about the benefits of breast feeding.

SOURCE: Frontiers, news release, Sept. 5, 2025