Breast pumps and baby formula: Does the time of day matter?

© 2010 Gwen Dewar, Ph.D., all rights reserved

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 just 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 6pm.

And when parents use baby formulas? They'll give their babies only special types for use at night--formulas that include sleep-inducing food additives.

This is a future that some researchers in Spain foresee. Their research suggests that breast milk contains substances that make babies sleepy—if that milk is produced in the evening or night-time.

What are these soporific substances?

Tryptophan and circadian rhythms

One possibility is the amino acid tryptophan. Tryptophan is used by the body to manufacture melatonin, the hormone that makes us feel drowsy.

Tryptophan is found in breast milk, but the amount depends on the time of day. Tryptophan levels rise and fall according to a circadian rhythm, with concentrations peaking at night.

So it seems plausible that babies who consume tryptophan in breast milk would show similar circadian rhythms in their melatonin production. Each night, babies consume tryptophan in breast milk. Then their bodies use this tryptophan to make melatonin.

There is some supporting evidence for this. Javier Cubero and colleagues studied 8 exclusively-breastfed infants, tracking changes in their mothers’ breast milk and changes in the infants’ melatonin levels.

The researchers also examined a control group of 8 formula-fed infants (Cubero et al 2005).

In both groups, babies were 12 weeks old. They were fed every 4 hours.

The results? For breastfed babies, melatonin levels peaked a few hours after their mothers’ tryptophan levels did.

By contrast, formula-fed babies--who didn’t get any “mother-made” tryptophan--peaked much later. And compared with the breastfed babies, formula-fed 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.

Other sleepy ingredients

Tryptophan isn’t the only substance that may help babies develop better sleep patterns.

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 parents should avoid giving “morning” milk to their babies 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--leaving aside the question of tryptophan and nucleotides--there may be other ingredients in breast milk that influence sleep.

For instance, in the morning most people experience peak levels of cortisol, a stress hormone. Does an alert morning mother produce the milk that is less conducive to sleep? It’s a reasonable possibility.

The bottom line?

If can do so without creating more hassles for yourself, it might be a good idea to avoid giving your baby “morning” milk at night.

And baby milk manufacturers should keep abreast of the new studies. Perhaps--someday--chrononutrition will lead to better baby formulas.

More information about breastfeeding, breast pumps, 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.



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References: Chrononutrition, breast pumps, and baby formula

Aparicio S, Garau C, Esteban S, Nicolau MC, Rivero M, and Rial RV. 2007. Chrononutrition: use of dissociated day/night infant milk formulas to improve the development of the wake-sleep rhythms. Effects of tryptophan. Nutr Neurosci. 2007 Jun-Aug;10(3-4):137-43.

Cubero J, Valero V, Sánchez J, Rivero M, Parvez H, Rodríguez AB, Barriga C. 2005. The circadian rhythm of tryptophan in breast milk affects the rhythms of 6-sulfatoxymelatonin and sleep in newborn. Neuro Endocrinol Lett. 26(6):657-61.

Cubero J, Narciso D, Terrón P, Rial R, Esteban S, Rivero M, Parvez H, Rodríguez AB, Barriga C. 2007. Chrononutrition applied to formula milks to consolidate infants' sleep/wake cycle. Neuro Endocrinol Lett. 28(4):360-6.

Cubero J, Chanclón B, Sánchez S, Rivero M, Rodríguez AB, Barriga C. 2009. Improving the quality of infant sleep through the inclusion at supper of cereals enriched with tryptophan, adenosine-5'-phosphate, and uridine-5'-phosphate. Nutr Neurosci. 12(6):272-80.

Harada T, Hirotani M, Maeda M, Nomura H, and Takeuchi H. 2007. Correlation between breakfast tryptophan content and morning-evening in Japanese infants and students aged 0-15 yrs. J Physiol Anthropol. 26(2):201-7.

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