A good night's sleep helps youngsters learn new words and remember words, finds research.
The findings could help teachers neuro-developmental disorders such as autism and dyslexia, both linked to poor sleeping and language problems.
Academics from York and Sheffield Hallam universities spent weeks coaching 53 primary school pupils aged seven to 12 at three boarding schools in North Yorkshire in language skills from lights out to breakfast time.
For 30 to 60 minutes a day, the youngsters were fed a diet of made-up words - similar to real ones but unique for research purposes.
Some were given the new words in the morning and tested on them the same day 12 hours later, by which time many had forgotten most of what they had learned.
But those given the words in the evening and asked to recall them the following morning did much better. Equally, those taught and tested in the same day who forgot the new vocabulary also found the words came back to them after a night's sleep.
"These are truly exciting results which open up a new dimension of research in our understanding of language development," the paper quoted Anna Weighall, from the psychology research group at Sheffield Hallam, as saying.
Words can get muddled up in the mind because they sound similar to others, the researchers said, but sleep cures this as well.
"Previous studies of toddlers assume word learning happens straightaway but the new findings suggest it takes longer," Dr Weighall added.
It appeared children use the same learning mechanism as adults - and sleep helps, she added.
Dr Lisa Henderson, of the Department of Psychology at York, added: "Children's ability to recall and recognise new words improved approximately 12 hours after training, but only if sleep occurs."
Professor Gareth Gaskell, also from York University, said: "Clearly, children need to learn material well in the first place, but then they also need to sleep well in order to weave these new memories in with their established knowledge."
The study, funded by the Leverhulme Trust, was published in Developmental Science.