Even facts "forgotten" by people during a busy day may be retrieved¹ if this is followed by a good night's sleep.

Researchers from the University of Chicago asked volunteers to remember simple words.

Many found their memories letting them down towards the end of the day, but the following morning, those who had slept well could recall much more.

Researchers, writing in the journal Nature, said the brain could "rescue" lost memories during the night.

When the brain is first asked to remember something, that memory is laid down in an "unstable²" state, meaning that it is possible that it could be lost.

At some point, the brain consolidates³ those it deems important into a "stable", more permanent state.

However, the Chicago researchers suggested that it was possible for a "stable" memory to be made "unstable" again.

This would mean that memories could be modified then filed away again in the face of new experiences.

The 12 volunteers tested in the experiment were played words created through a speech synthesizer which were purposely difficult to understand.

Initially⁴, the written version of the word was available, but afterwards the volunteers were asked to identify the word from the audio version only.

Tests revealed that the ability to recall the right word tended to tail off as the day ended.

However, when the volunteers were retested after a good night's sleep, they were able to recall some words that they had "forgotten" the previous evening.

Dr Daniel, one of the study authors, said: "Sleep consolidates memories, protecting them against subsequent interference or decay.

"Sleep also appears to 'recover' or restore memories."

He said: "If performance is reduced by decay, sleep might actively⁵ recover what has been lost."

Dr Karim Nader, from the Department of Psychology⁶ in McGill University in Montreal, said: "Memory research is undergoing a transformation⁷ - no longer is memory thought to be a hard-wiring of the brain, instead it seems to be a process of storage and re-storage.

"Sleep helps some memories 'mature' and also prunes⁸ out unimportant memories."