Breast milk is dynamic and constantly changing. It offers complete nutrition and hydration to your baby and lays the foundation for their immune system. Packed with vitamins, minerals, hormones, live cells, healthy bacteria (probiotics), and food for our good gut bacteria (prebiotics), it gives the baby everything they need for the first six months of life to grow their brain and body.

Breast milk changes during the different stages of lactation, from feeding to feeding and day to night. It responds to information from the baby and mother and adjusts to protect the baby from illness and fight off unwanted germs.

Babies are not born with a circadian rhythm of day and night. They get lots of information from their environment and the mother’s milk to develop their internal clock over time. Babies' sleep cycle matures and is a developmental milestone, not something we need to teach, nor can we teach. It will happen when they are ready and over time.

Exposure to light and dark helps inform you and your baby’s internal clock. We are hardwired to get sleepy when the sun goes down and more alert when the sun is up. Artificial light can interfere with our natural rhythms. Try keeping light low and limiting light exposure at night, and get plenty of sunlight with your baby during the day.

• The hormone cortisol is three times higher in your milk in the morning than it is late in the day. Cortisol is the hormone released during stress and aids in wakefulness. A baby doesn’t begin to have a cortisol circadian rhythm until about 8 weeks old. (1) 
• Melatonin peaks in the evening. Melatonin makes the baby more sleepy. It increases when it is dark. Keeping lights dim or dark during the night means your milk continues to produce milk higher in melatonin. (3)
• Adenosine, guanosine, and uridine are three nucleotides in breast milk that are highest between 8 pm and 8 am. They calm the nervous system, helping the baby to relax and fall asleep. (2)

Prolactin is the hormone that signals more milk to be made. It begins to rise after the baby starts to suck. As milk is emptied from the breast, the pituitary gland releases prolactin, which tells the cells in the mammary gland to draw nutrients from the blood to synthesize milk and store it for when the baby feeds again. The more often the baby nurses, the more often this signaling occurs, and the more often milk is made. 

Prolactin levels have their own circadian rhythm and are highest in the middle of the night, peaking between 2-5 am. When milk is removed in the middle of the night when prolactin is already elevated, it protects the mom’s overall milk supply. When prolactin levels are raised from the baby nursing regularly, including during the night, you are more likely to continue to experience lactational amenorrhea. (4)

During the late afternoon and early evening hours, when the baby cluster feeds, the milk-making hormone prolactin is at its lowest. Your breasts are naturally less full, and the baby will ask to feed more frequently. Older babies who are busy discovering their world and likely more distracted nurse for shorter periods of time and will make up for it by nursing more during the nighttime, further protecting mom’s milk supply. In fact, babies get about 20% of their calories from the milk they drink during the night.

Human milk is relatively low-fat but makes up about half the calories your baby takes. (5) It doesn't have a circadian rhythm, yet it is higher during the early evening hours vs early morning. When the breasts are less full, the fat content in milk is higher. (6) Going longer stretches between feeding or pumping, the breast will have more water content when the baby begins to nurse before more fat is mixed into the milk. You might notice when you feel less full of milk during periods of cluster feeding. Your baby gets more fat in the milk during those feeds, and they have a longer stretch of sleep afterward. Fat content in milk is highest between 12-6 pm. (7)

During the daytime, antibodies and white blood cells appear to be highest in breast milk. Cytokines are a group of proteins that control the immune response and are highest in milk during the day. (8) This means a lot of communication is passed between mom and baby through her milk, protecting the baby from illness.

Milk in the morning has higher magnesium, zinc, potassium, and sodium, while nighttime milk is higher in Vitamin E. Iron is highest at noon. (9)

Get to know the early cues of when your baby is ready to eat and offer before they show late feeding cues, especially crying. A baby crying when they are hungry is equivalent to when you wait too long to eat and become “hangry.” You know that more frantic, annoyed, angry feeling you get, and you just start grabbing whatever is in sight to eat rather than choosing your meal wisely, sitting down and eating it while relaxed, taking time to chew your food, and eating more slowly. When your baby cries because they are hungry, they have a more challenging time coordinating their feeding, may take in more air, or have less comfortable digestion.

When you follow your baby’s cues and practice side-lying bottle feeding, a style that is more baby-controlled than parent-led and a slower way to feed, they can pay attention to their satiety. Some feeds may be smaller, like 1.5-2 oz, while others will be 3-4 oz. They will not be on a set schedule but able to follow their natural hunger, so times between feedings will also vary.

By following their cues, you allow them to follow their natural biological rhythm, and you’ll know they are getting what they need from your milk when they need it.

The more we learn about the rhythms of all the components of breast milk, the more we may want to consider following Mother Nature’s lead. If you pump milk for your baby, it is recommended to label your milk with the time of day or night that it was pumped. 

For nighttime bottles, give your baby milk that was pumped at night and the same for milk pumped during the day. In this way, your baby naturally gets milk to keep their body in sync, the same as if they were directly feeding at the breast. Milk pumped during the day will deliver the ingredients to your baby that stimulate activity, and the milk pumped at night, if given at night, will help them relax and drift off to sleep.

Breast milk is unique, dynamic, and constantly changing. It is a  source of complete nutrition, hydration, and immunity, containing a wide range of nutrients, hormones, live cells, healthy bacteria, and prebiotics, giving babies everything they need for their first six months of life. Breast milk changes during the different stages of lactation, from feeding to feeding, from day to night, responds to the baby's and mother's information, protects the baby from illnesses and unwanted germs, and helps them to understand day from night.