Melatonin is the hormone produced by the pineal gland that plays a role in regulating sleep and circadian rhythm as well as a possible role in gut-brain signaling.[1] It is a normal component of breastmilk, with concentrations higher during nighttime (peak around 3 am) than daytime.[2-6] Women who work the night shift have lower milk melatonin concentrations during the midnight to 6:30 am period than on days when they are not working, which becomes larger on subsequent days of working the night shift.[7] Elective cesarean section results in higher daytime colostrum levels than with vaginal delivery.[8] Some authors suggest that mothers should nurse in the dark at night in order to avoid reductions in the melatonin content of breastmilk, which could disturb infant sleep patterns.[9] Differentiating milk pumped during the day from milk pumped during darkness has also been suggested for women pumping milk for their infants.[2,10] Some studies have attributed longer sleep time in breastfed infant than in formula-fed infants to melatonin in breastmilk.[11,12] Another study found higher colostrum melatonin levels at night which appeared to increase the phagocytic activity of colostral cells against bacteria.[13] A survey of 329 mothers found that infants who consumed mistimed expressed breastmilk took longer to get to sleep compared with infants who were directly breastfed, formula fed, fed timed expressed breast milk and fed breast milk/formula combined. Breastfed infants had more awakenings at night compared with infants who consumed mistimed expressed breastmilk.[14]

Exogenous administration of melatonin has no specific use during breastfeeding and no data exist on the safety of maternal use of melatonin during breastfeeding. However, doses higher than those expected in breastmilk after maternal supplementation have been used safely in infants.[15] It is unlikely that short-term use of usual doses of melatonin in the evening by a nursing mother would adversely affect her breastfed infant, although some authors recommend against its use in breastfeeding because of the lack of data and a relatively long half-life in preterm neonates.[16,17]

Dietary supplements do not require extensive pre-marketing approval from the U.S. Food and Drug Administration. Manufacturers are responsible to ensure the safety, but do not need to prove the safety and effectiveness of dietary supplements before they are marketed. Dietary supplements may contain multiple ingredients, and differences are often found between labeled and actual ingredients or their amounts. A manufacturer may contract with an independent organization to verify the quality of a product or its ingredients, but that does not certify the safety or effectiveness of a product. Because of the above issues, clinical testing results on one product may not be applicable to other products. More detailed information about dietary supplements is available elsewhere on the LactMed Web site.