Trials with 216 babies up to 12 months old indicated they were unable to remember new tasks if they did not have a lengthy sleep soon afterwards.
The University of Sheffield team suggested the best time to learn may be just before sleep and emphasised the importance of reading at bedtime.
Experts said sleep may be much more important in early years than at other ages.
People spend more of their time asleep as babies than at any other point in their lives.