Higher cortisol and cortisone concentrations in human milk were associated with more infant sleep after feeding, and more infant sleep before milk collection was associated with higher milk glucocorticoid concentrations, with no associations found between milk glucocorticoids and infant crying.
Key Findings
Results
Higher cortisol and cortisone concentrations in human milk were associated with more infant sleep in the 1.5 hours after a milk sample, contrary to the study's hypothesis.
The study hypothesized that higher cortisol and cortisone would be related to less infant sleep, but the opposite pattern was found.
Associations were examined using multilevel models accounting for the nested structure of repeated measures within mothers.
Sample consisted of 109 healthy mothers at 6 weeks postpartum.
Three milk samples were collected per day: morning, afternoon, and evening.
The 1.5-hour post-sample interval was used to assess effects of milk glucocorticoids on subsequent infant behavior.
Results
More infant sleep in the 1.5 hours before a milk sample was associated with higher cortisol and cortisone concentrations in the subsequent milk sample.
This finding suggests a bidirectional relationship between infant sleep and milk glucocorticoid levels.
The 1.5-hour pre-sample interval was used to assess how prior infant behavior related to subsequent milk glucocorticoid concentrations.
Both cortisol and cortisone showed this association, not just one glucocorticoid.
This represents a novel direction of inquiry as prior studies had not examined how infant behavior immediately precedes changes in milk composition.
Results
No associations were found between milk glucocorticoid concentrations and infant crying duration.
The study hypothesized that higher cortisol and cortisone would be related to more infant crying, but this was not supported.
Crying was assessed both in the 1.5 hours after and before each milk sample, as well as across the complete interval between samples.
This null finding contrasts with some prior literature associating elevated milk cortisol with more infant negative affect.
The absence of association held for both cortisol and cortisone.
Methods
The study employed a preregistered, within-day repeated-measures design to assess moment-to-moment bidirectional associations between milk glucocorticoids and infant behavior at 6 weeks postpartum.
N = 109 healthy mothers participated at 6 weeks postpartum.
Three milk samples were collected per day (morning, afternoon, evening), and mothers kept a logbook on infant crying and sleep the same day.
Three time intervals were analyzed: the complete interval between samples, the 1.5 hours after each sample, and the 1.5 hours before each sample.
Multilevel models were used to assess bidirectional associations, with both cortisol and cortisone examined separately.
The study was preregistered, indicating a priori hypotheses and analysis plans.
Background
Human milk glucocorticoid concentrations show diurnal variation and are impacted by both maternal and infant factors, motivating within-day repeated sampling.
The study collected samples at three time points within a single day to capture diurnal patterns.
Both cortisol and cortisone were measured as glucocorticoids of interest.
Prior evidence on milk glucocorticoids and infant behavior was characterized as inconsistent.
The bidirectional design was chosen because infant behavior may also influence milk composition.
Discussion
The study is described as the first to assess moment-to-moment associations between milk glucocorticoids and infant behavior, finding a pattern opposite to prior hypotheses regarding sleep.
Authors note this study 'broadens our understanding of the role of human milk composition on infant behavior and vice versa.'
The unexpected direction of the sleep findings (more sleep associated with higher glucocorticoids rather than less) challenges assumptions derived from stress-reactivity frameworks.
The bidirectional nature of findings suggests infant behavior and milk composition mutually influence each other on a short timescale.
The results highlight the importance of assessing direction and timing when studying milk composition and infant outcomes.
What This Means
This research suggests that the relationship between stress hormones (glucocorticoids, specifically cortisol and cortisone) in breast milk and infant behavior is more complex and even opposite to what scientists previously expected. The study tracked 109 mothers and their infants at 6 weeks after birth, collecting breast milk samples three times in one day and having mothers log how much their babies cried and slept. Contrary to the hypothesis that higher stress hormones in milk would lead to babies sleeping less and crying more, the study found that higher glucocorticoid levels in milk were actually linked to more infant sleep in the 90 minutes following a feeding. No link was found between milk glucocorticoids and infant crying at all.
The study also found that this relationship goes both ways: when babies slept more in the 90 minutes before a milk sample was collected, that milk tended to have higher glucocorticoid concentrations. This suggests that infant behavior itself may influence the composition of the milk that follows, not just the other way around. This bidirectional finding is notable because most prior research only looked at whether milk composition affects babies, not whether babies' behavior also shapes milk composition.
This research matters because it challenges simple assumptions about stress hormones in breast milk being harmful or disruptive to infant sleep. The findings suggest that glucocorticoids in human milk may play a more nuanced role in regulating infant sleep rhythms, and that studies need to carefully consider timing and direction when examining how milk composition and infant behavior are related. Future research will be needed to understand the biological mechanisms behind these associations.
Bruinhof N, Beijers R, de Weerth C. (2026). Moment-to-moment bidirectional associations between human milk glucocorticoids and infant behavior.. Psychoneuroendocrinology. https://doi.org/10.1016/j.psyneuen.2026.107790