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.
Human breast milk is more than a meal – it is ALIVE and it communicates to your babies body the time-of-day, vital stem cell information for self healing, fat volume increase for growth etc.
The composition of breast milk changes across the day, giving energizing morning milk a different cocktail of ingredients than soothing evening milk.
Breast milk may help program infant circadian rhythms, helping to explain why some parents of newborns enjoy long full nights of sleep, whereas others struggle to get their infants on a schedule.
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 This information is crucial for both breastfeeding and formula feeding families. 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.
What happens when babies drink night milk in the morning, or morning milk in the late afternoon? We don't know for sure, because this question has been woefully understudied.
Offering an infant a bottle of morning milk in the evening, with its high cortisol and low melatonin, might be the nutritional equivalent of flipping the lights on right before bedtime.
If chronosignals in milk do indeed help to calibrate infant circadian biology, then infants who drink "mistimed" milk may struggle more with sleep, digestion and development.
Labeling your pumped milk & coordinate feedings can help eliminate these issues.
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.
Riordan,J. and Wambach, K. Breastfeeding and Human Lactation. Boston, MA: Jones and Bartlett Publishers, 2010; 120-128. 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.
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. 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. Engler, 730.
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. 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.
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.
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. 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.
Darby Saxbe, USC Dornsife College of Letters, Arts and Sciences, Jennifer Hahn-Holbrook, University of California, Merced
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