Even facts "forgotten" by people during a busy day may be retrieved 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 "unstable" state, meaning that it is possible that it could be lost.

At some point, the brain consolidates 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.

Initially, 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 actively recover what has been lost."

Dr Karim Nader, from the Department of Psychology in McGill University in Montreal, said: "Memory research is undergoing a transformation - 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 prunes out unimportant memories."

如果晚上睡个好觉，即使白天由于忙碌而“遗忘”的一些事情也可以重新回忆起来。

芝加哥大学的研究者们要求志愿者们记住一些简单的单词。

到了晚上，许多人对自己的记忆力都感到失望。但是第二天早晨，那些睡得好的人能回想起来更多的单词。

研究者们在《自然》杂志发表论文说：大脑可以在晚间“修复”失去的记忆。

当大脑最初被要求记住一些东西时，那段记忆处于一种“不稳定的”状态，这意味着它很可能会丢失。

在有些时刻，大脑会将那些它认为重要的记忆强化为“稳定的”状态，也就是更持久的状态。

然而，芝加哥大学的研究者们提示说“稳定的”记忆也可能会转变为“不稳定的”。

这就意味着当遇到新的情况时，记忆可能会被修改，随后重新排列。

参加试验的12名志愿者收听了通过语音合成器产生的特别难懂的单词。

刚开始还给志愿者提供单词脚本，但是后来就要求他们仅仅通过声音来辨别单词。

实验显示：一天快结束的时候，准确回忆单词的能力随之减弱。

然而，当志愿者们睡了个好觉过后再次接受测试的时候，他们能够回忆起一些他们前一天晚上已经"遗忘"的单词。

这篇论文的一位作者丹尼尔博士说：“睡眠可以巩固记忆，防止它们随后受到干扰或衰减。

“睡眠看来也可以恢复记忆。”

他说：“如果记忆功能因为衰减而降低，睡眠可以有效的恢复丢失的记忆。”

蒙特利尔市麦基尔大学心理学系的卡里姆·纳德博士说：“记忆力研究正在转型——记忆不再被认为是大脑的硬件，而似乎是一个储存和再储存的过程。

“睡眠使某些记忆变得‘成熟’，同时也删掉了那些不重要的记忆。”