"When we planned the study, we worked with the hypothesis of association between heavy maternal consumption of caffeine and higher infant awakenings at night," Marlos Rodrigues Domingues, a researcher at Brazil's Universidade Federal de Pelotas and co-author of the study, tells Shots in an email.
It's not clear why the infants' sleep wasn't affected. The babies might have developed a tolerance to caffeine while in the womb, Rodrigues says. But other studies have found no caffeine metabolites in the urine of babies whose mothers drink coffee, suggesting that the babies don't absorb caffeine the way older children and adults do.
Image: Cup of coffee, via Shutterstock.