Babies who drink expressed breast milk can have disrupted sleep patterns if they consume the milk at a different time to when it was pumped. This may be due to them receiving a mistimed dose of hormones that influence sleep-wake cycles.
Infants are born without circadian rhythms. They rely on external cues, such as routines and changes in light levels, to help them develop a sense of night and day.
A group of German researchers previously found human breast milk , defined as 10pm to 10am, compared with during the day, 10am to 10pm. During the day, melatonin levels are replaced with the hormone cortisol, which promotes alertness.
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at La Trobe University in Bendigo, Australia, and her colleagues wondered whether drinking milk during the day that was pumped in the evening, and vice versa, could affect a baby鈥檚 sleep-wake cycle.
The team surveyed 329 women in Australia about how they fed their babies, who had an average age of 24 weeks, and how the infants slept. The researchers then compared the sleeping habits of the babies who were only directly breastfed, only formula-fed, mostly fed 鈥渢imed鈥� expressed breast milk or mostly fed 鈥渕istimed鈥� breast milk.
Women who expressed milk recorded whether the time they pumped fell between 6am to midday, midday to 6pm, 6pm to midnight or midnight to 6am. Mistimed milk was defined as the babies consuming the milk in a different window to the one it was expressed in.
The babies who received mistimed expressed milk took 15 minutes longer, on average, to fall asleep at night than the other groups, according to results presented at the Australasian Sleep Association meeting in Brisbane in November.
This may be because they were consuming breast milk that had been expressed during the day that was low in melatonin and rich in cortisol, says Booker. These babies also slept more than 40 minutes longer during the day than the other groups, possibly because they were drinking melatonin-rich milk that had been expressed in the evening, which made them groggy, says Booker.
The study was relatively small and relied on questionnaires. The researchers hope to repeat the research with a larger group of volunteers and the use of sleep monitoring devices to record the infants鈥� sleep patterns more precisely.
According to Booker, it is important to better understand whether drinking mistimed expressed milk disrupts circadian rhythms in infants, since research suggests these disruptions in adults 鈥� for example via shift work or jet lag 鈥� can lead to irritability, weight gain and other health issues.
Before storing expressed breast milk in a fridge or freezer, parents could label bottles with the time the milk was pumped, so it can later be given to infants at a similar time of day, she says.
In the US, . We don鈥檛 know much about the overall health effects of drinking expressed breast milk compared with direct breastfeeding, says at the University of California, Merced.
A study led by Hahn-Holbrook that randomly assigned women to feed their baby expressed breast milk or to directly breastfeed , possibly because this released hormones in their brains that promote bonding.
