Breast milk saved to give in a bottle should be given to a baby at the same time of day it was expressed, a new study said.

Researchers said that the levels of some components of breast milk change over a day. So, for example, milk expressed at night could help newborns go to sleep, they wrote.

Scientists at the University of Extremadur in Spain looked for three nucleotides in breast milk that excite or relax the central nervous system. In a study of 30 women, they found the highest levels in milk that came at night.

"This made us realize that milk induces sleep in babies", said Cristina L. Sanchez, lead author of the article. "You wouldn't give anyone a coffee at night, and the same is true of milk -- it has day-specific ingredients that stimulate activity in the infant, and other night-time components that help the baby to rest."

"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", points out the researcher," she said.

The study was published in the journal Nutritional Neuroscience.