by Katherine Brind Amour
October 13, 2009

A recent study by a team of Spanish researchers has identified yet another natural component of breast milk that distinguishes it from formula. Researchers from the University of Extremadura in Badajoz, Spain examined the breast milk of 30 mothers. Prior to feeding, each mother collected six to eight milk samples over a 24-hour period. Researchers then measured the level of three nucleotides—adenosine, guanosine, and uridine, in each of the milk samples.  Nucleotides store and transmit genetic information and play an important role in metabolism.

Results published in Nutritional Neuroscience showed that breast milk produced during the evening and at night contained more sleep-inducing nucleotides than milk produced during daylight hours.  Researchers also found that certain nucleotides, including some that have been shown to induce sleep, demonstrated a circadium rhythm, increasing with the onset of darkness.

It is widely accepted that the composition of breast milk changes over time, but how those changes affect infants are less clear.  There is speculation that these changes may occur in response to the changing developmental needs of infants as they transition from more frequent feedings and shorter sleep intervals to less frequent feedings and longer periods of sleep. The researchers also theorized that the cyclical variation in the concentration of nucleotides may explain the calming, sedating effect breastfeeding can have on babies, particularly at night.

Breastfeed at night
Although the study’s researchers have suggested that it would be a “mistake” to feed a baby expressed milk collected at a different time of day, the recommendation needs to be put in perspective.  Regardless of the potential variations in sleep-inducing nucleotides found in breast milk, the other benefits that breast milk confers when compared to artificial formula still make human milk the optimal source of nutrition for infants. In addition, the direct effect of specific nucleotides on the development of an infant’s sleep cycle and his or her adjustment to a circadian rhythm is as yet unclear.

While it might prove to be ideal to feed babies expressed milk at the same time of day that it was expressed, in practical terms doing so may not be easy. Many mothers express small amounts of breast milk throughout the day to complete a single feeding. While others express whenever time permits and not necessarily on a fixed schedule. What’s most important is that babies be breastfed or be given breast milk feedings whenever possible.

If further data show that the circadian rhythm in nucleotides significantly impacts the development of healthy sleep patterns, perhaps women will make more of an effort to offer expressed milk at the same time of day it was expressed.

Human milk has already demonstrated its capacity to cater to an infant’s needs, even from hour to hour. The fact that a mother’s milk is tailored to the nutritional needs and even the sleep patterns of the baby supports the growing body of evidence in favor of breastfeeding.

Until a connection is demonstrated between nucleotide circadian rhythms and the health of the baby, mothers who pump breast milk and later bottle-feed their infants should not worry about feeding milk pumped at a different time of day. Rather, mothers can take comfort in knowing that by breastfeeding at night, they are lulling their babies to sleep naturally and safely.