Research: Breast milk helps set baby's clock

An evening feeding might send ‘time for bed’ signals from mom to baby.

I have two children. The first one was breastfed until two-and-a-half years old. The second one is now 22 months old, still being breastfed. The first one slept overnight at four months old. The second one at six months old.

When I share this experience with people, they often stare at me, “and they’re breastfeeding babies? Impossible!”

I never knew how to explain that. People seem to believe that formula fed babies sleep longer and better, as my mom, my in-laws and my postpartum doula insisted.

But recent study helped me answer that question: human breastmilk may help babies tell time via circadian signals from mom!

It turned out that breastmilk contents keep changing based on the time of the day. Just like adults often decide on the specific combination of ingredients they want for a cocktail depending on mood or occasion, 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 may take several months to one year to develop circadian rhythms, depending on the environment they are in.

A mother’s milk follows a daily cycle or circadian rhythm; breast milk provides “chrononutrition” and may set baby’s clock.

The researcher found that 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 the 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.

Now I have the perfect answer for those who (including myself) wonder how my breastfeeding babies slept over night by six months old. Or, I’ll just share with them this cute video produced by USC that explains the research in one minute!


This article is republished from San Diego County Breastfeeding Coalition's Blog by To-wen Tseng. Photo credit to the author.