Breast pumps can be used any time, but the composition of breast milk changes throughout the day. Might the timing influence baby sleep? It's possible.
Some researchers call it “chrononutrition,” the idea that we can improve our sleep cycles and other daily rhythms by adjusting the timing of what we eat.
For example, some food ingredients make us more alert. Others make us sleepy.
So maybe it will become standard operating procedure:
Mothers who use breast pumps and store their breast milk will note the time of day that their milk was expressed. When they feed their babies at night, they’ll use only milk that was produced after dark.
And when parents use baby formulas? They'll give their babies only special types for use at night, formulas that include sleep-friendly food additives.
This is a future that some researchers foresee. A team in Israel tracked changes in the breast milk of five women, testing milk content every 2 hours for a 24-hour period. In particular, they measured levels of melatonin, the hormone that makes us feel drowsy after dark.
Were there any differences between milk expressed during the day and milk expressed in the evening? Yes, there were, and the researchers found evidence of a clear, circadian rhythm. Milk produced at night contained higher levels of melatonin (Cohen Engler et al 2012). It's not yet clear if fluctuating levels of melatonin in breast milk influence baby sleep patterns. Future studies may provide answers.
Other researchers have focused on the amino acid tryptophan. Tryptophan is also found in breast milk, and it is used by the body to manufacture melatonin. Like melatonin, tryptophan levels rise and fall according to a circadian rhythm, with concentrations peaking at night.
Do babies who consume tryptophan in breast milk experience a rise in melatonin levels? There is some evidence to support the idea.
Javier Cubero and colleagues studied 8 exclusively-breastfed infants, tracking changes in their mothers’ breast milk and changes in the infants’ melatonin levels. They also monitored a control group of 8 formula-fed babies (Cubero et al 2005).
All the babies were approximately 12 weeks old, and they were each fed on the same schedule -- once every 4 hours. But there was a difference between groups. For breastfed babies, melatonin levels peaked a few hours after their mothers’ tryptophan levels did. For formula-fed babies, the peak in melatonin happened much later, and infants spent significantly less time sleeping at night.
Did the tryptophan in breast milk cause differences in melatonin rhythms and sleep patterns?
That’s not clear. Possibly, the link between maternal tryptophan and infant sleep patterns was due to chance. And even if it wasn’t, we can’t be sure about causation.
Maybe something other than maternal tryptophan could explain why breastfed babies were different. For example, breastfed babies might have gotten more skin-to-skin contact, which lowered their stress levels and made it easier to sleep. Or maybe the breastfeeding mothers in this study were more likely to expose their babies to natural lighting patterns, which would have helped their babies adjust to the 24-hour day.
So I’m interested in a study conducted by Sara Aparicio and colleagues (Aparicio et al 2007). These researchers tested the effects of dietary tryptophan by performing a double-blind experiment on formula-fed babies (Aparicio et al 2007).
There were three conditions:
Test condition: Babies got standard formula during the day and tryptophan-enriched formula at night.
Control condition 1: Babies got standard formula, day and night.
Control condition 2: Babies got tryptophan-enriched formula during the day (6am to 6pm) and regular formula at night (6pm to 6am).
After a week on the “night-time tryptophan” regimen, babies in the test condition showed improvements in their sleep patterns. Babies in the control groups did not.
So it seems that higher tryptophan concentrations--which are typical of night-time breast milk--may indeed help young babies sleep better.
Melatonin and tryptophan aren't the only sleep-associated substances in breast milk. Cristina Sanchez and her colleagues note that several nucleotides—like 5’UMP, 5’AMP, and 5’GMP—either induce sleepiness or help regulate circadian rhythms (Sanchez et al 2009).
These nucleotides are present in breast milk. Do their concentrations in breast milk change over a 24-hour period? The researchers asked 30 women to express their breast milk at several different times of day. Then the milk was analyzed.
Nucleotide concentrations in breast milk did indeed change over a 24-hour period.
For example, 5’UMP--which has a calming effect--peaked in the middle of the night.
And 5’AMP, which makes people feel drowsy, was at its highest concentration in breast milk that was expressed in the early night-time.
Can we assume that nucleotides in breast milk might help infants fall asleep—and stay asleep—during the night?
To answer this, we need another experiment like that one conducted on formula-fed babies by Sara Aparicio’s team. Some experiments have been conducted, but they involved food that was fortified with both tryptophan and sleep-friendly nucleotides (Cubero 2006; Cubero et al 2007). As a result, we don’t know what effect the nucleotides might have had on their own.
Does this mean that chronologically "mismatched" milk could make babies too sleepy in the morning -- or too wired at night?
Keep in mind that these studies were small and leave many questions unanswered. More research is needed, and the reality is bound to be complex.
For example, tryptophan doesn’t just help the body produce melatonin. It’s also important for the production of serotonin, a neurotransmitter that helps regulate mood and cognitive functions during the day. If you consume tryptophan in the morning, it's likely to get converted into serotonin. Later in the day, your body will break down some of the serotonin and use the components to make melatonin.
So seems unlikely that boosting levels of dietary tryptophan in the morning would interfere with an infant’s developing sleep rhythms. Indeed, a recent study in Japan suggests that kids who consume more tryptophan at breakfast have better sleep rhythms (Harada et al 2007).
But mothers also pass stress hormones through their milk, and there is evidence that stress hormone levels in milk have an impact on infant behavior (Hinde et al 2015; Grey et al 2013; Glynn et al 2007). People experience surges in the stress hormone, cortisol, during acute stress. They also experience, normal, healthy peaks in cortisol in the morning. If mothers pump milk at a time when their cortisol levels are high, might they be producing the sort of milk calculated to keep babies alert, restless...perhaps even a bit irritable? It's a reasonable question.
For more information about the composition of breast milk, click here.
For more breastfeeding information, including tips about breast pumps, see these practical breastfeeding tips.
Struggling with a sleepless baby? Check out these evidence-based articles about the science of baby sleep.
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Hinde K, Skybiel AL, Foster AB, Del Rosso L, Mendoza SP, and Capitanio JB. 2015. Cortisol in mother’s milk across lactation reflects maternal life history and predicts infant temperament. Behavioral Ecology 26 (1): 269-281.
Sánchez CL, Cubero J, Sánchez J, Chanclón B, Rivero M, Rodríguez AB, and Barriga C. 2009. The possible role of human milk nucleotides as sleep inducers. Nutr Neurosci. 12(1):2-8.Content of "Breastfeeding and baby formula" last modified 4/10