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Engineering Your Milk

Updated: Oct 7, 2021

I've had moms ask me a lot this month about melatonin in milk pumped at night, and whether they should separate out (label) their day milk and night milk to promote night sleep in baby. They've been reading this on the Internet and wonder if this is a "thing".

I've also seen some posts on social media recently how the American Academy of Pediatrics (AAP) changed their recommendations when it comes to adding warm milk to cooled milk, and how it's now OK to "pool" milk into one pitcher to decrease the nutrient and caloric variability of milk pumped throughout the day. Sounds like conflicting advice, right? Should I pool my milk to "even out" the nutrients and calories or should I separate it out based on the time of day it was pumped to give baby the most melatonin for night feedings?

Now, if you're exclusively nursing, of course this isn't a "thing" you have to think about at all. You just lift your shirt and your body does the work of warming the milk and providing the r

ight fats/nutrients/melatonin. But if you're regularly pumping, these recommendations might be something to look at (IF you are so inclined). I'm one who doesn't like to make breastfeeding and/or pumping complicated. We honestly don't have to overthink things as much as we sometimes do (especially with the Internet making us second guess ourselves on the daily).

So let's look at each one individually, shall we?


It indeed is a "thing" that mother's milk is higher in melatonin (a compound that makes one sleepy, helps aid digestion, and promotes cell restoration) in the middle of the night (peaking around 3 am). It's also true that babies do not make their own melatonin until about 3 months of age. We also know many babies do not sleep through the night in the early months, right? And formula-fed infants never get ANY melatonin from their mothers (yet they somehow sleep at night). So, is melatonin from mother's milk absolutely essential to help babies sleep? This has not been proven (only hypothesized), so if separating out and labeling night vs day milk is troublesome and more work for you (like makes you want to bag it all and switch to formula), then you do not necessarily HAVE to do that. If you have the time and want to try it and have the mental bandwidth for this, then go for it. But remember, even the authors of this study admit:

"More evidence is needed to support the labeling of expressed milk as morning, afternoon, evening, or night milk or the overall concept of circadian-matched feeds. Similar to the paper on circadian variation of breast milk nutrient composition, we found that potential confounders included maternal dietary intake, maternal nutritional status such as body mass index, and other factors that contribute to maternal chronodisruption, such as insomnia or work and study schedules that involve being awake throughout the night" (Moyo et al., 2021).

And this:

"Randomized controlled trials are needed to determine whether providing circadian-matched milk improves child health" (Hahn-Holbrook et al, 2019).

Indeed mother's milk is incredible, and there is still SO MUCH mystery about how the baby interacts with and benefits from the breast and breastmilk that it blows my mind. I love reading about the science of it all, but when it comes to putting research into recommendations, I think we have to be careful what sort of extra homework we are assigning to our poor parents who are already doing the work of pumping and bottling (and washing everything).

The researchers have highlighted important variances in breastmilk, and this is a good first step, but more research with clinically meaningful outcomes needs to be realized before making recommendations that impact parent's lives (IMO). With that being said, "You do You" and perform your own little experiment with the labeled melatonin milk (or not) and let me know what you discover.


I've seen some buzz recently regarding how the American Academy of Pediatrics (AAP) changed their recommendations when it comes to adding warm milk to cooled milk, and how it's now OK to "pool" milk into one pitcher to decrease the nutrient and caloric variability of milk pumped throughout the day.

As far as I can tell, these aren't "new" recommendations, as the study they cite on their FAQ is from 2013 (8 years ago!), so I'm a bit confused by this….but be that as it may, some bloggers are pointing out that this is a new recommendation, much to the relief of all parents who have all along been adding warm to cooled milk and pooling it all together! Whew. And to make things even MORE confusing, if you go to scroll down on the AAP's FAQ page you'll find the Centers for Disease Control and Prevention (CDC) link which takes you to a page that says NOT to mix cold with warm milk. Eekk. Who should you believe? What should you do?

I've never seen any compelling evidence saying that it is dangerous to add warm milk to cooled, or to pool old milk with new milk (as long as the oldest milk is not past the expiration date, of course). So, Yay! If your baby is not premature or immunocompromised, and you start with clean containers and pump only after washing your hands, your warm milk added to the cold should be just fine. There are macrophage in your freshly expressed milk that will most likely gobble up any bacteria they come into contact with in the cooler milk, so BOOM. No need to worry about bacterial overgrowth affecting your milk. Common sense, though: if the older/cooler milk is getting close to 4 days old, freeze it that day (label the bag with the date of the oldest milk).

Got more questions? Let me know if I can answer them.



Hahn-Holbrook, J., Saxbe, D., Bixby, C. et al. Human milk as “chrononutrition”: implications for child health and development. Pediatr Res 85, 936–942 (2019).

Moyo, G., Thomas-Jackson, S., Childress, A., Dawson, A., Thompson, L., Oldewage-Theron, W. (2021). Chrononutrition and Breast Milk: A Review of the Circadian Variation of Hormones Present in Human Milk. Clinical Lactation Journal, 12(3), 114-121.

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