The study was conducted by the University of Oregon, and included 20 babies between six and 12 months old. They were taken to the lab each night to sleep in an MRI scanner.
While they were sleeping, researchers had someone read neutral nonsense phrases in different tones of voice.
Alice Graham, a researchers, said, "Even during sleep, infants showed distinct patters of brain activity depending on the emotional tone of voice we presented."
Babies who came from homes where arguments happened often seemed to be the most sensitive to voice tones, as their brains reacted more readily to anger.
Another study conducted by British researchers at King's College London discovered similar things, including that sleeping babies can detect when their parents are upset or angry.
In the study the babies were also placed in an MRI scanner, but instead of saying phrases in different tones of voice, the team played a variety of human vocalizations like laughing and crying. The sounds of crying resulted in more activity in the same part of the brain that is active when adults hear sad sounds.
Lead doctor in the study, Declan Murphy, said, "You'd think that a sleeping baby wouldn't be able to hear you, but clearly their brains are processing sounds while they're asleep."