Breast milk is an amazing and adaptive substance that changes throughout the day.
Breast milk is a living organism with over 700 living bacterial species. It changes during a feeding, with the watery foremilk coming out first, followed by the richer, creamier hindmilk.
However, there is little scientific information on the way in which breast milk changes throughout a 24-hour cycle. Experts are now beginning to discover that the fat content and levels of certain key nutrients and hormones in breastmilk vary with the mother’s circadian rhythm.
A paper titled, “Circadian Variation of Breast Milk Components and Implications of Care” was published in the Journal Breastfeeding Medicine. In it, Dr. Robert White, a neonatologist from Beacon Children’s Hospital, highlighted the idea that the components of breast milk change throughout the day in response to the mother.
The fat content of breast milk is typically higher in the evenings, which some argue helps a newborn to stay satiated and sleep for longer periods of time at night. Additionally, levels of tryptophan, an amino acid used to make the hormone melatonin, are present in higher amounts at night. Studies have found other vitamins and hormones, such as iron and cortisol, vary according to the circadian rhythm as well.
Dr. White points out that when a mother is pregnant, infants experience the same day-night cycles as their mothers do, including temperature changes, the mother’s physical activity, and the placental transfer of hormones, such as cortisol and melatonin. Upon birth, babies don’t initially produce the hormones needed to direct their own circadian rhythms. However, these circadian stimuli may continue to be received through breastfeeding.
Melatonin and various proteins that are the building blocks of other hormones are present in breast milk in varying amounts throughout a 24-hour period. As such, breastmilk may influence a newborn’s sleep-wake cycle. White indicates that further research is needed to determine whether or not infants who receive expressed breast milk would benefit from consuming the milk at the same time of day that it was expressed.
While more research needs to be done surrounding the topic of breast milk and the circadian rhythm, it is safe to conclude that breast milk is unique in its ability to adapt to the day-night cycle.