Does the composition of breast milk change over the course of the 24-hour day? Yes. Breast milk pumped at night has more melatonin — the hormone of drowsiness — and higher levels of nucleotides thought to promote sleep. By contrast, breast milk pumped in the morning features higher concentration of cortisol, a stress hormone that makes babies more alert.
So if you pump and store breast milk, it’s probably a good idea to label it with the time of day it was produced, and then offer your baby milk that is “circadian-matched.” Researchers suspect that “mistimed” milk could contribute to sleep troubles, and possibly disrupt your baby’s developing circadian rhythms.
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. And breast milk? It might be able to do both, depending on when it’s produced.
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? Maybe they’ll use slightly different formulations depending on the time of day — to mimic the natural changes in breast milk.
Why exactly might this be a good idea? In part, it has to do with the “drowsy” hormone, melatonin.
Consider, for example, a study of five breastfeeding mothers in Israel. Researchers tracked changes in breast milk over time, testing milk every 2 hours for a 24-hour period. In particular, the researchers 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 concentrations of melatonin (Cohen Engler et al 2012).
Subsequent research has confirmed these results. For instance, in a study of 21 lactating mothers living in Germany, researchers didn’t just find that melatonin levels were higher at night. They were 5 times higher than daytime levels (Katzer et al 2016).
Similarly, researchers in China have documented dramatic changes across a 24-hour period. Yishi Qin and her team tested the breast milk of 98 lactating mothers, and compared milk melatonin levels at 3pm, 9pm., and 3am (Qin et al 2019). On average, melatonin levels at 9pm were nearly 3 times as high as melatonin levels at 3pm. And the breast milk pumped in the middle of the night? At 3am?
That’s when melatonin concentrations were at their peak. Breast milk pumped at 3am had nearly 10 times as much melatonin as breast milk pumped during the afternoon.
There’s also evidence regarding the stress hormone cortisol: It reaches peak concentrations in morning breast milk.
Cortisol helps make us feel alert, and it’s possible that babies could receive a boost in cortisol depending on the timing of breast milk production.
Morning levels are approximately 4 times higher than levels present in breast milk produced in the evening (around 6pm). And they are about twice as high as levels present in milk expressed during the night (Pundir et al 2017; Italianer et al 2020).
Then there’s the research regarding the sleep-related effects of tryptophan.
Tryptophan is 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 (Caba-Flores et al 2022). 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 controlled experimental studies, like one conducted by Sara Aparicio and colleagues (Aparicio et al 2007). The researchers tested the effects of dietary tryptophan by performing a double-blind, randomized trial on formula-fed babies (Aparicio et al 2007).
There were three conditions:
- In the test condition, babies got standard formula during the day and tryptophan-enriched formula at night.
- In control condition #1, babies got standard formula, day and night.
- In 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 daytime tryptophan concentrations might help young babies sleep better.
Other sleepy ingredients?
Melatonin and tryptophan aren’t the only sleep-associated substances in breast milk. Researchers note that several nucleotides—like 5’UMP, 5’AMP, and 5’GMP—either induce sleepiness or help regulate circadian rhythms (Caba-Flores et al 2022; Sanchez et al 2009).
These nucleotides are present in breast milk. Do their concentrations in breast milk change over a 24-hour period? Cristina Sánchez and her colleagues 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 “mistimed” milk could affect an infant’s behavior?
More research is needed to answer this question, and the reality is bound to be complex. But as Jennifer Hahn-Holbrook and her colleagues have noted, it’s certainly plausible.
Animal studies confirm that the melatonin in breast milk “rapidly crosses the intestinal barrier and diffuses into many tissues, including the liver, kidney and brain” (Hahn-Holbrook et al 2019). It’s also clear that glucocorticoids (like cortisol) can pass from breast milk into a young rodent’s brain, and we’ve good reason to think that the same is true for humans (Hahn-Holbrook et al 2019).
Moreover, researchers have documented a correlation between “mistimed” milk and sleep. Lauren Booker and her colleagues conducted a survey of more than 329 mothers, and found that mistimed, expressed breast milk was significantly associated with babies falling asleep later at night (Booker et al 2022).
So it seems wise to assume that the timing of breast milk production could make a difference, and perhaps one that goes beyond the minutes or hours immediately after a baby feeds. Hahn-Holbrook and her colleagues point out that a regular diet of “mistimed milk” might interfere the development of a baby’s circadian rhythms, contributing to sleep problems. It might also alter an infant’s stress responses.
The good news? If mistimed milk really does present problems, these are easy to fix. We just need to note the time of day when milk is expressed, and avoid giving our babies milk that is chronologically mismatched.
More information about feeding, breast milk, and baby sleep
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.
References: Does the timing of breast milk affect infant sleep?
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Content of “Breastfeeding and baby formula” last modified 2/2023 Most of this text derives from an earlier version of the same article, published under the former title, “Melatonin, breast milk, and breast pumps: How the timing of milk production might affect infant sleep.”
Title image of infant feeding, mother concealed face by Nattakorn Maneerat / istock
image of mother bottlefeeding infant, sideview by PorporLing / shutterstock
image of father bottlefeeding infant by Halfpoint / shutterstock