Summary: Breast milk may help train an infant’s circadian clock. The hormonal and nutritional composition of milk changes by time of day: cortisol tends to be higher in the morning and melatonin rises in the evening and overnight.
Source: The Conversation
Human breast milk is more than nutrition — it also conveys time-of-day cues. The makeup of breast milk shifts across the day, so morning feeds contain a different blend of hormones and nutrients than evening feeds. Researchers call this “chrononutrition,” and they suspect it helps shape infants’ developing circadian systems, the internal clock that teaches babies to tell day from night.
But what happens when babies are given milk that was expressed earlier or at a different time of day than when it is fed? With widespread use of breast pumps and refrigerated storage, milk is often pumped and then provided hours later. Scientists have rarely studied whether these “mistimed” feeds influence infant development, even though the potential implications are large.
As psychologists focused on the biology of parenting, we joined colleagues to examine the evidence that breast milk carries signals for infants’ daily rhythms.
Body clocks over the course of the day
Sleep, appetite and energy follow circadian rhythms—daily patterns driven by internal clocks and environmental cues like light and dark. Newborns do not arrive with fully organized day–night cycles; these rhythms emerge over the first weeks and months of life as infants learn to respond to cues such as exposure to daylight and nighttime darkness.
Infants vary widely in how quickly they develop stable rhythms. Some show consistent daily patterns in hormones tied to alertness and sleep and sleep long stretches early on. Others take months to establish regular cycles and may appear rhythmically “upside-down.” Delays in circadian development can relate to problems such as prolonged fussiness, digestive disturbances, or difficulties with feeding and growth.
Why circadian maturation differs so much from one infant to another is not well understood. One promising explanation is that breast milk itself helps program these rhythms, which could account for part of the variation parents observe in newborn sleep and feeding patterns.
Milk in flux
Breast milk composition changes systematically across the day. Hormones that promote wakefulness and alertness, like cortisol, are generally higher in milk expressed in the morning than in milk expressed in the evening. By contrast, melatonin — a hormone that supports sleep and digestion — is typically low in daytime milk and rises in the evening, peaking around midnight.
Beyond hormones, other milk components also vary by time of day. Certain nucleotides that may support sleep are found at higher levels in night milk, while some amino acids associated with activity are more abundant in daytime milk. Iron levels in milk often peak around midday, and vitamin E tends to be higher in evening milk. Minerals such as magnesium, zinc, potassium and sodium have been reported at higher concentrations in morning milk. Immune factors, including some antibodies and immune signaling molecules, can also differ between day and night samples.
We know that these hormones and immune factors are transferred from mother to infant during feeding, and we also know that infants are actively developing their own circadian regulation in the first months. It is therefore plausible that the time-varying signals in breast milk help shape infants’ emerging daily rhythms. Differences in feeding schedules, and whether infants receive milk that matches the current time of day, might help explain why some babies establish regular sleep–wake and feeding patterns sooner than others.
Mistimed messages in milk?
Historically, breast milk was consumed as it was produced, so infants always received milk that reflected the immediate time of day. Modern practices — pumping, storing, and later feeding — mean a baby might receive morning-expressed milk at bedtime or night-expressed milk in the morning. This raises the possibility that such “mistimed” milk could send mixed signals to an infant’s developing clock.
Little research has directly tested whether feeding milk expressed at a different time of day affects infant sleep, digestion or developmental outcomes. Conceptually, though, offering milk high in morning cortisol during an evening feed (when melatonin would normally be higher) could be akin to exposing a baby to light at night—potentially disrupting the settling of circadian rhythms.
There are practical steps that could be taken if time-of-day signals in milk prove important. Parents can label pumped milk with the time it was expressed and aim to match morning milk to morning feeds, afternoon milk to daytime feeds, and night milk to evening or overnight feeds. Neonatal intensive care units could adopt time-matching as part of broader efforts to support circadian development, alongside practices such as dimmed lighting at night. Milk banks might also consider organizing donor milk by time of day.
An alternative public-health approach is to support parental leave and workplace policies that make it easier for mothers to breastfeed directly, reducing the need for pumped and stored milk. Direct breastfeeding at the time milk is produced avoids the issue of mistiming and may provide additional health benefits.
Ongoing research will clarify how breast milk’s time-dependent signals influence infant biology and whether matching milk to the clock can improve sleep, digestion and longer-term development. If time-matched feeding proves beneficial, the case would strengthen for policies and clinical practices that help mothers feed infants in alignment with natural daily rhythms.
Funding: Darby Saxbe receives funding from the National Science Foundation.
Source:
The Conversation
Media Contacts:
Darby Saxbe & Jennifer Hahn-Holbrook – The Conversation
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The image is adapted from The Conversation news release.