Briana Tillman, Colorado Springs, Colorado, USA

Photo:Shutterstock/Leonid and Anna Dedukh

Human milk’s complex adaptations are amazing. Breast milk undergoes biochemical changes during a feeding; the consistency develops visually over the course of a pumping session from watery foremilk to creamier hindmilk. Human milk also has the ability to provide for an infant’s changing nutritional needs with age. The percentages of fat and protein levels change over time to meet the increased energy demands of mobile babies.(1) If these changes in human milk don’t impress you, recent research suggests that its composition shifts in synchrony with mothers’ circadian rhythms as well1, giving breastfed babies a head start in neurological development and sleep patterns.

Bleary-eyed lactating mothers might be surprised by reports that breastfed babies sleep, on average, 45 minutes longer per night and experience less colic than their formula-fed peers.(2) Variables affecting infant sleep are notoriously difficult to measure in isolation, but in recent years some scientists have begun to look at breast milk biochemistry as a potential source of “chrononutrition”2. Interesting findings include several substances that vary in accordance with the mother’s circadian rhythms, including some amino acids, melatonin, trace elements, and even a few nucleotides—the building blocks in many important biological processes.

In 2008, Spanish researchers took breast milk samples from 77 women in three-hourly increments and measured the levels of 16 amino acids.(3) Of these, four amino acids that are precursors to activity neurotransmitters (“wakefulness” amino acids) were found to peak during the daytime and reach their lowest levels at night. Conversely, tryptophan, a precursor to melatonin infamous for causing drowsiness, peaks during the night. While these amino acid circadian rhythm indicators are not present during the colostrum phase (with the exception of tryptophan), they do seem to help inform the newborn’s neurological development during transitional and mature milk periods.

Another Spanish study confirmed the circadian rhythm of tryptophan in breast milk and carried the research further by studying the levels of a melatonin metabolite

(breakdown product) extracted from the urine in breastfed infants’ diapers.(4) The diaper study was worth the trouble; not only did the melatonin metabolite show clear circadian rhythms in the breastfed babies, but its rhythm was also linked to the mothers’ levels of tryptophan. The graphs are striking in their synchronicity, with babies’ levels showing a slight delay to allow for construction and metabolism of melatonin before its metabolite is excreted.

Important precursors to melatonin follow circadian rhythms in breast milk, but the hormone itself is also present and increases during periods of darkness.(5)  This is especially important during the first few weeks of life, when babies are not yet making their own supply of melatonin. Since the hormone serves both a hypnotic role and also relaxes the gastrointestinal muscles, breast milk melatonin could be a major factor in early neurological development of sleep/wake cycles, as well as reduced colic incidence.

Scientists have found other substances in breast milk that follow circadian rhythms, but the purposes or causes remain unclear. Iron, for example, peaks at noon, vitamin E peaks at about 6 pm(6), and magnesium and zinc are both at their highest levels in the morning.(7)  Sodium and potassium also follow predictable variations in breast milk during the day(8), but neither the mechanism nor the possible impact of these changes is yet understood. Fat content shows significant changes during the night(9); this may, however, be related to changes in frequency of feeding rather than circadian rhythms brought about at a cellular level.(10) In short, researchers are just beginning to discover the many new implications of day/night variation in breast milk.

For breastfeeding mothers and La Leche League Leaders, there are two clear messages:

1. Formula can’t compare

The uniqueness of breast milk as individually tailored for normal neurological and biological development remains unchallenged by any formula company in the world. Formula does not include melatonin and other important chrononutritive components, and no formula has yet achieved circadian rhythms in composition.

2. Breast milk affects a baby’s sleep patterns

Labeling pumped milk with the time of day collected may help maintain the valuable connection between breast milk components and infant sleep even when mother has to be away.

As scientists continue to explore the biochemical makeup of human milk, we may find new keys to unlocking the secrets of chrononutrition, neurological development, and hormonal activity. Until then, continuing to support breastfeeding mothers improves the health of future generations in many ways, some of which we are only beginning to understand.


1A circadian rhythm can be likened to an internal body clock influencing sleeping and eating patterns over 24 hours.
2Chrononutrition is an eating plan developed by a French nutritionist that emphasizes consuming certain nutrients at different times of the day.

1. Riordan,J. and Wambach, K.  Breastfeeding and Human Lactation. Boston, MA: Jones and Bartlett Publishers, 2010; 120-128.
2. Engler, C. et al. Breastfeeding may improve nocturnal sleep and reduce infantile colic: potential role of breast milk melatonin. European Journal of Pediatrics 2012; 171(4):720-32.
3. Sanchez, C. et al. Evolution of the circadian profile of human milk amino acids during breastfeeding. Journal of Applied Biomedicine 2013; 11(2):59-70.
4. Cubero, J. et al. The circadian rhythm of tryptophan in breast milk affects the rhythms of 6-sulfatoxymelatonin and sleep in newborns. Neuro Endocrinology Letters 2005; 26(6):657-61.
5. Engler, 730.
6. Barkova, E.N. et al. Diurnal variations in qualitative composition of breast milk in women with iron deficiency. Bulletin of Experimental Biology and Medicine 2005; 140(4):394-6.
7. Karra, M.V. and Kirksey, A. Variation in zinc, calcium, and magnesium concentrations of human milk within a 24-hour period from 1 to 6 months of lactation. Journal of Pediatric Gastroenterology and Nutrition 1988; 7:100-106.
8. Keenan, B.S. et al. Diurnal and longitudinal variations in human milk sodium and potassium: Implication for nutrition and physiology. American Journal of Clinical Nutrition; Mar 1982; 35(3):527-34.
9. Lubetzky, R. et al. Circadian variations in fat content of expressed breast milk from mothers of preterm infants. Journal of the American College of Nutrition 2006; 25(2):151-4.
10. Daly, S.E. et al. Degree of breast emptying explains changes in the fat content, but not fatty acid composition, of human milk. Experimental Physiology 1993; 78: 741-755.

Briana TillmanBriana Tillman became an LLL Leader in 2008, and started the first La Leche League Group on the Korean peninsula. Since then she has served as a Leader in Germany, and her current Group is in Colorado Springs, Colorado, USA. Briana became an International Board Certified Lactation Consultant (IBCLC) in 2014 and plans to begin medical school later this year. Briana has three children, aged nine, seven, and four years.

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