The composition of breast milk changes significantly to fit the needs of a baby over a 24-hour period, so mothers should not express breast milk at one time and use it at a different time, according to a new study by Spanish researchers.

Milk expressed in the morning typically contains nucleotides and other ingredients that excite a baby's central nervous system, while milk expressed at night has a different combination of nucleotides that promote relaxation and sleep, the scientists found.

"It is a mistake for the mother to express the milk at a certain time and then store it and feed it to the baby at a different time," said Cristina Sanchez, a researcher at the chrononutrition laboratory at the University of Extremadura, in Badajoz, Spain.

Sanchez, in a new study published in the journal Nutritional Neuroscience, collected breast milk expressed over a 24-hour period from 30 women. The highest nucleotide concentrations were found in the night-time samples (8 p.m. to 8 a.m.), which, Sanchez said, "made us realize that milk induces sleep in babies." 

You wouldn't give coffee to someone at night, she said, "and the same is true of [breast] milk. It has day-specific ingredients that stimulate activity in the infant, and other night-time components that help the baby to rest."

Jim Dawson reports for Inside Science News Service.