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Breast Milk Keeps Time and May Set Baby’s Clock

Breast milk contents keep changing based on the time of day.

Photograph by Rama, Wikimedia Commons, Cc-by-sa-2.0-fr
Source: Photograph by Rama, Wikimedia Commons, Cc-by-sa-2.0-fr

Adults know what a cocktail is and often decide on the specific combination of ingredients they want depending on mood or occasion. It turns out that a mother’s body manufactures cocktails for a breastfeeding baby—cocktail ingredients that change throughout the day.

New parents know that babies’ bodies do not necessarily follow a daily cycle or circadian rhythm, most obviously when a baby is awake at night and sleeping by day. Babies’ bodies take several months to develop circadian rhythms, helped by the environment they are in.

How babies learn circadian rhythm is built into humanity’s evolved nest, specifically breast milk taken from the breast. A mother’s milk follows a daily cycle or circadian rhythm. Breast milk appears to provide “chrononutrition.”

Here are quotes from a recent press release from researchers of a new study:

“Breast milk changes dramatically over the course of the day. For example, levels of cortisol—a hormone that promotes alertness—are three times higher in morning milk than in evening milk. Melatonin, which promotes sleep and digestion, can barely be detected in daytime milk, but rises in the evening and peaks around midnight.

Night milk also contains higher levels of certain DNA building blocks which help promote healthy sleep. Day milk, by contrast, has more activity-promoting amino acids than night milk. Iron in milk peaks at around noon; vitamin E peaks in the evening. Minerals like magnesium, zinc, potassium, and sodium are all highest in the morning.

Daytime milk may pack a special immune punch. Among mothers who provided researchers with milk samples across the first month postpartum, immune components—including key antibodies and white blood cells—looked higher in day milk compared to night milk. Another study found higher levels of a component important for immune system communication in day milk compared to night milk.”

Mother’s milk, directly from the breast, may help the baby’s body develop a circadian rhythm: the ability to recognize night and day and behave accordingly.

Although researchers know very little about how the changes in breast milk affect infant health, this research suggests that if a mother pumps milk in the morning and gives it to a baby at night, it will have the wrong cocktail of ingredients for promoting sleep (and promote wakefulness instead).

A study conducted over 10 years ago found that over 85 percent of U.S. mothers pumped breastmilk.

The rate is likely to be higher now as working mothers have increased since that time.

As the authors of the press release point out, if pumped breast milk is time-sensitive, then parental leave policies are critical health initiatives.

What is a breast-pumping mother to do?

  • Label the pumped milk by the time of day.
  • Breastfeed directly at night to ensure that sleep-inducing ingredients are provided.
  • Take a leave from work so you can feed the baby directly instead of pumping.

It's National Breastfeeding Month. Learn more.

More from Darcia F. Narvaez Ph.D.
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