A good night's sleep is all it takes to bolster your memory, a team of scientists has found.

Not only does it help strengthen your memory, it can help you recall memories you thought you had lost for good, they say.

To test the effect of sleep on memory, Dan Margoliash and a team at the University of Chicago trained students to recognise words spoken from a speech synthesizer. The machine had been set up to make words such as dog, cat and big almost, but not quite, incomprehensible.

After a training session in the morning the students were, on average, 20% better at recognising the garbled words than they were before training.

But Professor Margoliash found that when the same students were tested later in the day, their memories had started to fail them. "They were only half as good later in the day," he said.

But when they were tested after a night's sleep, their performance had shot back up. "What they had lost during the day, they regained after a night's sleep," Prof Margoliash said, adding: "Sleep had a whopping effect."

Sleep has long been thought to play a key role in the way the brain consolidates experiences into memories, but until now there was no evidence that it could help recover lost memories.

"I think what is happening during sleep is that you are recreating the experience and by doing that you somehow fix the memory," Prof Margoliash said.

The study, which was carried out with Kimberly Fenn and Howard Nusbaum, also of the University of Chicago, appears in the journal Nature today.

Appreciating the role sleep plays in forming memories is crucial for anyone trying to learn something new.

"It is absolutely the case that if you want to learn something and remember it for a long time, then an integral part of your strategy has to be to get a good night's sleep," Prof Margoliash said.