It's no surprise human milk is amazing stuff! But, did you know your milk can help your baby learn to tell time? Not only does human milk composition change over the course of lactation from colostrum to mature milk to toddler milk and beyond, it also varies hour by hour over the course of a day. Your milk will provide what your baby needs for the specific time of day that your baby is breastfeeding. How cool is that? This circadian variation in hormone levels transfers information about time of day from mom to baby and known as chrononutrition.
Circadian rhythms are established in infancy and sleep-wake cycle is one component of this. Human milk plays an important role in sleep-wake cycles because the maternal hormones that fluctuate over the day are transmitted to the baby through breast milk. Human milk naturally has higher levels of cortisol in the morning when mom's cortisol levels are higher. This promotes alertness and feeding behaviors. During the night, milk has lower levels of cortisol and higher levels of melatonin which promotes sleep and restoration. When you feed directly from the breast, your milk will automatically promote regulated rhythms! Dimming the lights at night and feeding in a calm darkened environment is a great idea too! This not only provides environmental cues but promotes melatonin release from mom's brain and into milk where it will transmit to the baby.
But what about feeding expressed breast milk? Expressed breastmilk will still have varying levels of cortisol and melatonin. When you feed expressed breast milk, the time of expression will determine the hormone content of the milk. So, if you can, try to choose milk that was pumped around the time you are giving it to your baby. This is called circadian matching. For example, if you know you will supplement in the middle of the night, breast milk that you pumped during a previous night would be a great choice if you have it!
This "night time" milk will provide adequate levels of melatonin that will promote sleep in your baby. If you don't have time matched milk, please don't stress about it and feed your baby what you have. And remember, if you supplement for any reason any time of day, you should be pumping to replace the milk to maintain an optimum supply.
A word about milk pooling (mixing several pumping sessions of milk together)...you will maintain many benefits of breastmilk but you will lose the benefit of circadian matching. I don't recommend milk pooling for several reasons and this is one of them.
Pro Tip: If you are pumping to store breast milk, an easy way to designate "night time" milk is with a piece of blue painter's tape on the top of the bag (or you can designate a different brand bag for those nighttime pumps). This will make it easier to pick out from your freezer when you need it! And remember, always date and time your milk too as best practice.
In conclusion, human milk is continually changing to meet your babies needs and circadian matching can promote regulated sleep-wake cycles in your baby. Better sleep for baby means better sleep for you!
Please reach out if you have any questions and I hope you found this information helpful!
Family Nurse Practitioner/IBCLC
Walker, M (2021) Breastfeeding Management for the Clinician Using the Evidence. 5th ed. Jones & Bartlett Learning