By the time babies are a year old they can recognise a lot of sounds and even simple words, says Marie Cheour from the University of Turku in Finland. She suspects this might be because they learn language while they sleep as well as when they are awake.
To test the theory, Cheour and her colleagues studied 45 newborn babies in the first few days of their lives.
They exposed all the infants to an hour of Finnish vowel sounds - one that sounds like "oo", another like "ee", and a third boundary vowel peculiar to Finnish and similar languages that sounds like something in between.
Fifteen of the babies then went back with their mothers, while the rest were split into two sleep-study groups.
One group was exposed throughout their night-time sleeping hours to the same three vowels, while the others listened to other, easier-to-distinguish vowel sounds.
When tested in the morning, and again in the evening, the babies who had heard the tricky boundary vowel all night showed brainwave activity indicating that they could now recognise this new sound.
They could identify the sound even when its pitch was changed, while none of the other babies could pick out the boundary vowel at all.