Melatonin is a naturally occurring hormone produced by the pineal gland, often called the “darkness hormone” because its release is triggered by low light conditions. This substance regulates the body’s sleep-wake cycle, known as the circadian rhythm. For a nursing infant, melatonin is a time-sensitive biological signal transferred directly from the mother’s bloodstream into her breast milk. This rhythmic transfer carries significant implications for a baby’s early development and sleep patterns.
The Daily Timing of Melatonin Transfer
The production and subsequent transfer of melatonin into breast milk follows a predictable 24-hour cycle that mirrors the mother’s own internal clock. Melatonin levels begin to rise noticeably in the mother’s milk shortly after sunset, typically around 8:00 PM or 9:00 PM, as the body responds to decreasing light exposure. This increase signals the beginning of the “night milk” period, which is distinctly different in its hormonal profile from milk produced earlier in the day.
The concentration of melatonin in breast milk reaches its highest point in the late-night and early-morning hours, with research indicating the peak time falls consistently between 12:00 AM and 3:00 AM. At this peak, the melatonin levels can be nearly ten times higher than the concentrations found in milk collected during the afternoon. Conversely, milk expressed during the daytime, specifically between 10:00 AM and 4:00 PM, contains only negligible or barely detectable amounts of the hormone.
This fluctuation in breast milk composition is directly tied to the mother’s light environment, not the act of feeding itself. The hormone is synthesized in the mother’s body in response to darkness and then passively passes into the milk ducts from the maternal circulation. The resulting change provides the baby with a chemical cue that distinguishes day from night, delivering time-of-day information through nutrition.
How Melatonin Influences Infant Sleep Cycles
For the first three to four months of life, a newborn’s pineal gland is not mature enough to produce its own rhythmic pattern of melatonin secretion. The infant’s circadian rhythm is underdeveloped and relies heavily on external cues to become established. Ingesting melatonin-rich milk during night feedings supplies the hormone the baby cannot yet generate for themselves.
The melatonin consumed acts as a “chrononutrient,” a substance that helps to synchronize the baby’s emerging biological rhythms with the 24-hour cycle of light and dark. This hormone signals “nighttime” to the infant’s body, promoting drowsiness and helping to regulate the sleep-wake cycle. This transfer is thought to support the deep sleep that newborns often experience between 3:00 AM and 7:00 AM.
The day-to-night hormonal difference in milk also includes other substances that support this physiological signaling. For instance, the stress hormone cortisol, which promotes alertness, is found in much higher concentrations in morning milk. The balanced interplay between high morning cortisol and high nighttime melatonin provides the infant with clear, contrasting biological instructions for when to be awake and when to rest. This dynamic is thought to be essential for programming the infant’s own long-term circadian biology.
Applying Timing Knowledge to Feeding Routine
Understanding the time-sensitive nature of breast milk allows parents to implement “chrononutrition” when feeding expressed milk. This practice involves aligning the time the milk was pumped with the time the baby receives it to ensure the hormonal signals are appropriately timed. Feeding an infant milk pumped in the morning at a late-night feeding time may inadvertently provide low melatonin and high cortisol, which could be disruptive.
To maximize the benefits of the nightly melatonin surge, parents who pump can label their stored milk as “morning,” “afternoon,” or “evening”. The milk collected from late-evening and overnight pumping sessions should be reserved for the last feed before the baby’s longest sleep stretch. This ensures the baby receives the natural sedative signal just before they are expected to sleep.
Maternal environmental factors also influence the quantity of melatonin transferred to breast milk. Exposure to bright light, particularly blue light from electronic screens, can suppress the mother’s natural melatonin production. Limiting exposure to blue-wavelength light before the baby’s bedtime helps ensure the milk contains the highest concentration of the sleep-promoting hormone. Feeding the baby in a dark or dimly lit environment at night also supports the transfer by avoiding suppression of the mother’s nocturnal hormone levels.