Breast milk variations in melatonin and cortisol levels may impact babies’ circadian rhythm

Scientists have discovered that breast milk conveys signals that may influence babies’ circadian rhythms from mother to infant. However, certain breast milk hormones — particularly melatonin and cortisol — and proteins involved in this process vary in concentration over the course of a day.

The researchers advise timing each feeding of expressed milk to help maintain infants’ natural circadian signals that support sleep, metabolism, and immune development.

They highlight that not all mothers have time to breastfeed multiple times during the day and night. Many busy women often store expressed milk for later feeding times, which may significantly influence babies’ natural biological alignment.

“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,” advises senior author professor Maria Gloria Dominguez-Bello, researcher at the Department of Biochemistry and Microbiology at Rutgers University, US.

First author Dr. Melissa Woortman, a recent Ph.D. graduate from the Department of Nutritional Sciences at Rutgers, believes 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 believes the findings reinforce an understanding that breast milk is a “dynamic food.”

Dominguez-Bello adds: “The timing of these cues would be particularly critical in early life, when the infant’s internal circadian clock is still maturing.”

Spikes in circadian signaling

The researchers took 10 mL breast milk samples from 21 participants at 6 a.m., 12 p.m., 6 p.m., and 12 a.m. on two different days, which were about a month apart. Another 17 participants provided samples taken during the same times once, resulting in 236 samples in the analysis.

The samples were examined for levels of the hormones melatonin, cortisol, and oxytocin, as well as immunoglobulin A (IgA), an antibody protein in the immune system, and lactoferrin, a milk protein.

Melatonin peaked at midnight, whereas cortisol was highest in the early morning.

While these hormones are involved in regulating the circadian rhythm, the other analyzed breast milk components involved in intestinal development and gut microbiome dynamics were mostly stable throughout the day.

The study authors say this might be because these other compounds may not be as strongly influenced by signals that dictate circadian rhythms.

“When it comes to differences in day/night variations by infant age, this could reflect the stabilizing of the maternal circadian clock that occurs with time after giving birth, as well as the maturing and stabilization of the infant’s circadian rhythm,” says Woortman.

Changes throughout childhood

The team also found that as infants got older, the levels of different compounds in breast milk varied.

For example, the levels of cortisol, IgA, and lactoferrin were highest when babies were less than one month old. Higher levels of these compounds likely support immune defense and gut colonization in very young infants.

The researchers say their study does not account for all potentially relevant demographic factors, including delivery mode and maternal diet, because of its sample size. Larger and more diverse cohorts will be needed in the future to ensure the generalizability of these results.

Additionally, they say future research should examine how infants respond to the variations observed in this paper. Their findings are published in Frontiers in Nutrition.

Last month, global ministries of health convened on World Breastfeeding Week (August 1–7), highlighting that breastfeeding reduces the risk of childhood obesity even when a range of other potential factors are taken into consideration. 

Meanwhile, a recently launched clinical trial sponsored by the US National Institutes of Health is investigating whether mothers consuming peanuts and eggs during breastfeeding could be effective at protecting their babies from allergies to these foods.