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Even facts “forgotten” by people during a busy day may be got back again 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 remember 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 “unstable” state, which means it is possible that it could be lost.

At some point, the brain consolidates (强化) those which are thought 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 slightly changed and then be kept in a new order again in the face of new experiences.

The 12 volunteers who were tested in the experiment were asked to listen to words created through a speech synthesizer (语音合成器) which were purposely difficult to understand.

At the very beginning, the written version of the word was available, but afterwards the volunteers were asked to identify the word just depending on the sound they heard.

Tests showed that the ability to remember the right word tended to weaken 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 strengthens memories, protecting them against later interference (干扰) or decay (衰减).”

“Sleep also appears to ‘recover’ or restore memories.” He said “If the function of memory is reduced by decay, sleep might actively recover what has been lost.”

Dr Karim Nader, from the Department of Psychology in McGill University in Montreal, said “Memory research is experiencing a transformation -- no longer is memory thought to be a hardware of the brain, instead it seems to be a process of storage and re-storage. Sleep helps some memories ‘fully grown up’ and also removes unimportant memories.”