Ever watched a sleeping baby and thought that behind those peaceful brows their brain was processing information? No, me either.

I loved to watch my sleeping babies and enjoy their cuteness without any demands on me. It never occurred to me in these moments that their brains were processing their daily learning as they slept.

Everything is new for a baby. With each new experience, their brains have the job of organizing and categorizing all that information. It is thought that sleep is integral to this process.

A recent study found that six- to eight-month-old babies were able to create a meaning for a word, but only after they’d had a nap. Creating meaning for a word was previously only thought possible in older children and adults.

The researchers exposed six- to eight-month-old infants to fictitious objects, which they gave made-up names such as “Bofel” or “Zuser.” Objects that were similar but not the same were all called a “Bofel” or “Zuser” accordingly. The researchers used fictitious objects to make sure that the babies could not use their preexisting knowledge in the tests.

After the first learning phase in the morning, the babies’ brain responses showed that they did not recognize a new Bofel as a “Bofel,” although it was quite similar to the previously seen Bofel versions.

When tested again, after their nap, babies’ brains were able to differentiate between the right and wrong name for a new object. The babies could see the commonalities between the similar pairs, known as generalization.

It seems that babies had generalized their knowledge during their sleep, because babies that stayed awake during their nap were still unable to generalize their learning.

The amount and quality of the learning was dependent on the amount of sleep they got in their nap. Babies who slept for about 50 minutes showed a brain response called the N400 component that indicated to the researchers they had created meanings for the words. The N400 component is also found when older children and adults learn a meaning for a word.

Babies that slept 30 minutes or less, however, were only able to filter out the similar features of the contexts and link them to the sound of the word. They had not created a meaning for the word.

“Our results show that infants can form long-term memory for word meanings much earlier than previously thought. Even though the brain structures that are relevant for this type of memory are not fully matured, they can already be used to a certain extent,” explains Angela D. Friederici, senior author of the study.

The researchers say that one particular stage of sleep is responsible for the memory of word meaning. This is the second of the four sleep stages. This stage of light sleep allows the transition from simple early learning, like that seen in the babies who napped for 30 minutes, to an advanced form of memory as seen in the babies who napped for 50 minutes.

Compared to other types of development which can take months, this learning is happening in a matter of minutes, leading the researchers to deduce that language development appears to occur in “fast motion” during sleep.

“In our study, however, infants were exposed to a large amount of information within a narrow time frame, which they normally experience over a much longer period of time,” study leader Manuela Friedrich adds.

“But only during sleep, when the infant brain is isolated from the surrounding world, can it extract and save relations incorporated in this information. And only the interaction of an alert state of experiencing the environment with the offline state of sleep, in which experiences are organized and stored, enables early cognitive and language development.”

Next time you see a sweet sleeping bundle, think about all the development that’s going on inside that brain. All the words and activities you shared with them that morning are being categorized and developing meaning. It just goes to show that babies are amazing, even when they sleep.