Breast milk is a dynamic biological fluid whose composition constantly adjusts, including nutritional components, immune factors, and hormones, based on the time of day it is produced. This fluctuation, known as diurnal variation, reflects the mother’s internal biological clock. Parents who pump often ask if the sleep-regulating hormone melatonin is present in milk expressed at night. The answer is yes, and this temporal difference has implications for infant development and feeding practices.
The Role of Maternal Melatonin and Circadian Rhythm
Melatonin, often referred to as the “darkness hormone,” regulates the sleep-wake cycle in humans. In the mother, this hormone is synthesized by the pineal gland in response to darkness. As the mother prepares for sleep, circulating melatonin levels rise significantly, typically beginning in the evening hours. This increase facilitates the passive transfer of melatonin into the breast milk, following the maternal 24-hour circadian rhythm.
Studies show that the concentration of melatonin in milk is substantially higher at night compared to the daytime. Melatonin levels start to increase around 9 p.m. and usually reach their maximum concentration between 2 a.m. and 3 a.m. The peak concentration in night milk can be ten times higher than the nearly undetectable levels found during the afternoon. This creates a distinct biological difference between “day milk” and “night milk,” which also includes components like the stress hormone cortisol, which peaks in the morning.
Impact on Infant Sleep and Circadian Development
The presence of melatonin in night-time breast milk is important because a newborn’s internal clock is not yet fully developed. Infants are not born with an established circadian rhythm and typically do not produce their own rhythmic melatonin until three to four months old. For the first several months, the baby depends on external cues delivered through breast milk to organize internal processes.
The melatonin in night milk acts as a “chronobiotic signal,” communicating the time of day to the infant’s body. This hormonal signaling helps synchronize the infant’s suprachiasmatic nucleus (SCN), the master clock in the brain, with the external light and dark cycle. This process, known as circadian entrainment, contributes to promoting better sleep patterns.
This biological timing explains why feeding morning milk at nighttime can be counterproductive. Morning milk contains cortisol, which promotes wakefulness, while night milk contains sleep-inducing melatonin. Providing mistimed milk may disrupt the programming of the infant’s sleep architecture and potentially delay the establishment of their mature circadian rhythm.
Guidelines for Timing and Labeling Pumped Milk
Due to the difference in hormonal content, parents feeding expressed milk are advised to maintain natural time-of-day cues. This practice, sometimes called “chrononutrition,” maximizes the benefits of breast milk. The most effective strategy involves matching the time the milk was pumped to the time it is fed to the infant.
Parents should label all pumped milk clearly with the date and the specific time it was expressed, such as “AM Feed,” “Midday,” or “PM Feed.” For practical purposes, milk pumped between 9 p.m. and 3 a.m. should be reserved for night feedings, and milk pumped between 6 a.m. and noon should be used for daytime feedings. Melatonin levels remain stable and do not degrade when the milk is frozen, stored, and thawed.
For parents with a large stored supply, this labeling system ensures the baby receives the appropriate biological signal. Aligning the milk’s hormonal composition with the time of feeding supports the infant’s developing sleep, metabolism, and immune system. Timing expressed milk feedings is a straightforward way to continue providing these natural regulatory cues.