Summary: Breast milk provides more than calories and nutrients — it carries time-of-day signals that can influence infant sleep, immunity and metabolism. In an analysis of expressed milk, researchers found clear day–night variation in hormonal signals, with melatonin highest at night and cortisol peaking in the early morning, while many immune-related proteins remained relatively stable across the day.
The study also showed that newborns under one month receive higher concentrations of protective factors such as immunoglobulin A (IgA) and lactoferrin. These results suggest that matching the time a baby is fed expressed milk to the time it was pumped may help preserve the milk’s natural developmental cues.
Key Facts
- Circadian signals: Hormones in breast milk, notably melatonin and cortisol, follow daily rhythms.
- Age-dependent composition: Immune proteins like IgA and lactoferrin are most abundant in milk for infants younger than one month.
- Practical recommendation: Labeling expressed milk by the time it was collected could help caregivers maintain the milk’s natural timing signals.
Source: Frontiers
Breast milk is a dynamic biological fluid and a primary source of nutrition and immune protection for many infants. Beyond macronutrients and vitamins, it contains hormones and bioactive proteins that convey physiological information from mother to child and support early development.
Because many caregivers rely on expressed and stored breast milk, understanding how its composition changes over the day is important. If the concentrations of hormones and immune factors vary by time, feeding milk at a different time than it was produced could alter the signals an infant receives.
To investigate these daily fluctuations, researchers in the United States collected expressed breast milk samples across four time points in a 24-hour cycle and analyzed levels of several hormonal and immune components.
Their findings were published in Frontiers in Nutrition.
Lead author Dr Melissa Woortman, a recent PhD graduate from Rutgers University’s Department of Nutritional Sciences, noted that the study “observed measurable differences in breast milk bioactive components depending on the time of day, reinforcing that breast milk is a dynamic food.” Senior author Professor Maria Gloria Dominguez-Bello of Rutgers’ Department of Biochemistry and Microbiology added that the timing of these signals is likely important in early life, when an infant’s internal clock is still developing.
Study design and measurements
The researchers analyzed 236 milk samples collected from 38 lactating participants. Twenty-one participants provided 10-milliliter samples at 6:00 am, 12:00 pm, 6:00 pm and 12:00 am on two occasions about a month apart; another 17 participants provided samples at the same four time points once. The team quantified levels of melatonin, cortisol and oxytocin (hormones), plus immunoglobulin A (IgA) and lactoferrin (immune-related proteins). They also assessed the milk microbiome using sequencing methods.
Melatonin and cortisol are well known to follow circadian rhythms in maternal blood, and these hormonal patterns were reflected in milk. Melatonin concentrations were highest at night, with a peak around midnight, while cortisol levels were greatest in the early morning. Other measured components, including many immune proteins, showed much less variation across the day.
The relative stability of some immune factors may indicate they are regulated differently than hormones tied to circadian signaling, or that their roles require steady presence regardless of time of day.
Variation with infant age
The analysis also revealed that milk composition changes with infant age. Levels of cortisol, IgA and lactoferrin were higher in milk provided for infants younger than one month. The elevated presence of these compounds in early milk likely supports immune defense and intestinal colonization during the newborn period, when infants are most vulnerable and their own physiological systems are rapidly developing.
The authors suggest that some of the observed age-related differences may reflect postpartum changes in the mother’s circadian regulation as well as maturation of the infant’s own circadian rhythm.
Implications and practical advice
While the study could not account for every potentially relevant factor — such as delivery mode or detailed maternal diet — due to sample size and cohort characteristics, the results point to a simple practical step that may preserve milk’s natural timing signals: labeling expressed milk with the time of expression. By feeding ‘morning’ milk in the morning and ‘night’ milk at night, caregivers may help maintain the hormonal and microbial rhythms that support infant sleep patterns, metabolism and immune development.
As Professor Dominguez-Bello noted, in modern contexts where continuous direct breastfeeding may not be possible, aligning the timing of expression and feeding is a feasible strategy to maximize the benefits of expressed breast milk. Dr Woortman emphasized that further research with larger, more diverse cohorts is needed to confirm these findings and to explore how infants physiologically respond to milk collected at different times of day.
About this breastfeeding and circadian rhythm research news
Author: Deborah Pirchner
Source: Frontiers
Contact: Deborah Pirchner – Frontiers
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Day/Night Fluctuations of Breast Milk Bioactive Factors and Microbiome” by Melissa Woortman et al., Frontiers in Nutrition
Abstract
Day/Night Fluctuations of Breast Milk Bioactive Factors and Microbiome
Introduction: Human breast milk is a complex biological fluid delivering nutrition, immune components and microbes crucial for infant development. Because maternal serum hormones show circadian variation, assessing whether breast milk mirrors these rhythms is important, particularly when expressed milk is fed at times different from when it was produced.
Methods: The study measured 24-hour variations in milk composition by analyzing samples from 38 lactating mothers collected at four times of day. Concentrations of cortisol, melatonin, immunoglobulin A (IgA), lactoferrin and oxytocin were quantified, and milk microbiome composition was evaluated by 16S rRNA sequencing.
Results: Melatonin and cortisol showed significant day–night fluctuations. Levels of lactoferrin and IgA varied primarily with infant age, maternal body mass index and infant sex rather than consistently across the day. The milk microbiome also shifted, with nighttime increases in skin-associated bacteria and daytime rises in environmental bacteria, influenced by maternal BMI and infant age. Alpha diversity of the milk microbiota tended to increase with infant age.
Discussion: These daily differences in breast milk composition underline the physiological relevance of the milk’s temporal dynamics. Feeding expressed milk asynchronously from its time of expression may alter the natural timing cues infants receive, with potential implications for sleep, metabolism and immune maturation.