# The Circadian Composition of Breast Milk: A Natural Starting Point for Chrononutrition

## TL;DR

Breast milk changes throughout the day, offering different nutrients and hormones that may help regulate infants' circadian rhythms and development.

## Contribution

This review highlights the circadian composition of breast milk as a natural form of chrononutrition for infants.

## Key findings

- Night milk contains higher melatonin and tryptophan, while day milk has elevated cortisol.
- Breast milk's changing composition supports infants' sleep-wake cycles and metabolic programming.
- Diurnal fluctuations in nutrients and immunological components suggest a role in infant development.

## Abstract

Breast milk is a highly bioavailable biological fluid that provides all essential fluids, macro and micronutrients, growth factors, and bioactive substances necessary for optimal growth and development of infants. Chrononutrition is a concept that examines the effects of the timing of food intake on physiological rhythms. This review aims to summarize current evidence on the circadian composition of breast milk and its potential implications for infant chrononutrition and early-life metabolic programming.

Fluctuations in the composition of breast milk across a 24-hour period, particularly the variations between night and day milk, and infants’ consumption of milk with this changing composition, contribute to breast milk being considered a form of chrononutrition. Thus, infants affected by signals from their mothers during the prenatal period continue to regulate their circadian rhythms by connecting with their mothers after birth. The circadian variation in breast milk is best characterized by higher concentrations of melatonin and tryptophan in night milk and elevated cortisol levels in day milk. Beyond these neuroendocrine factors, the concentrations of macro and micronutrients and immunological components have also been shown to exhibit diurnal fluctuations.

These chrononutritional properties of breast milk may contribute to the regulation of infants’ sleep-wake cycles and support growth and development.