INTRODUCTION: The hormone melatonin regulates the sleep and this pineal hormone is synthesized in the organism from the amino acid tryptophan. It is known that breast-fed babies have better sleep patterns and a better entrained sleep/wake cycle than bottle-fed babies (adapted formula). OBJECTIVE: To compare the circadian rhythm of 6-sulfatoxymelatonin (aMT6s)–the metabolite of melatonin excreted in the urine–in urine of bottle-fed and breast-fed children, and relate it to the circadian rhythm of tryptophan in breast milk, also evaluating the possible effects on the baby’s night-time rest. METHODS: 16 infants of 12 weeks of age were studied, divided into two groups depending on their exclusively natural or artificial feeding. The circadian rhythm of 6-sulfatoxymelatonin in urine was measured for the two groups of infants and for the breast-feeding mothers. In the breast milk, the circadian rhythm of the amino acid tryptophan was measured. The rest of the infants was tested by wrist actimeters for a week and the sleep parameters of the infants were measured and evaluated.

RESULTS: The tryptophan in the breast milk presented a circadian rhythm with acrophase at around 03: 00. This affected the 6-sulfatoxymelatonin circadian rhythm with acrophase at 06: 00 in the breast-fed infants, and also promoted nocturnal sleep. Assumed sleep, actual sleep, and sleep efficiency were significantly increased in the breast fed infants with respect the forrmula fed infants. CONCLUSION: A temporal relationship was observed between the circadian rhythm of 6-sulfatoxymelatonin of the exclusively breast-fed babies and that of tryptophan in the mother’s milk. Acting this aminoacide as a zeitgeber entrainment of the biological rhythms in the breast-fed infant.