I recently came across a study in the journal "Nutritional Neuroscience" in which a group of researchers from Spain looked at the levels of certain substances (nucleotides) in breast milk, which are known to induce sleep. The researchers found that there was a definite pattern to when their levels were higher and lower, and that those known to induce sleep were, not surprisingly, higher in breast milk produced during the evening and nighttime than in breast milk produced during the day. In the paper (cited below), the authors also mention previous studies demonstrating that breast milk levels of melatonin, a hormone secreted prior to sleep onset, also vary in relation to the time of day, with melatonin found in breast milk produced at night, but undetectable in breast milk produced during the day.
Both of these findings, in addition to those of other studies mentioned in the paper raise questions about whether the common practice of new mothers to pump breast milk for later use, may have disruptive effects upon the organizing sleep patterns of their children. The rationale for pumping this is that it allows the mother to return to work while maintaining her ability to continue to produce milk despite long periods when she is unable to nurse, as well as being able to supply milk for the caregiver to feed the baby with. While there are compelling reasons for feeding an infant human breast milk, some of which have been discussed in previous posts, if the milk itself contains compounds which can induce (or ward off sleep), one has to ask if giving milk produced in the morning, to a baby in the early evening, might have negative effects upon the baby's sleep patterns, which in turn, may lead to both short and long term consequences.
I am certainly not suggesting that women should stop pumping breast milk and start their children on formula at age 2 months. However, it may be worthwhile to label the milk with the time it was produced so that it can later be given to the baby at approximately the same time of day, thereby reducing the potential for disruption of the developing sleep patterns.
Dennis Rosen, M.D.
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Sánchez CL, Cubero J, Sánchez J, Chanclón B, Rivero M, Rodríguez AB, Barriga C. The possible role of human milk nucleotides as sleep inducers. Nutr Neurosci. 2009 Feb;12(1):2-8.