While we doze, our brain busily squirrels away memories. But not just any memories – it turns out that during sleep the brain specifically preserves nuggets of thought it previously tagged as important.
Jan Born of the University of Tübingen in Germany and his colleagues asked 191 adults to perform different memory tasks, such as learning word-pairs. Half were told to expect a test on the task 9 hours later, while the others were told they would have a different kind of task. During the interval some members of each group were allowed to sleep.
Participants who went to bed anticipating a post-nap quiz recalled 12 per cent more word pairs than those who slept with no expectation of a test. Furthermore, those anticipating a test also experienced more slow-wave sleep, known to be linked to memory consolidation.
By itself sleep did not significantly improve memory – participants who were not anticipating a test performed just as badly as one another regardless of whether or not they'd had a nap before the exam.
The results improve our understanding of sleep, says Born. "There is an active memory process during sleep that selects certain memories and puts them in long-term storage."
The study is "very convincing", says Penny Lewis, who studies memory and sleep at the University of Manchester, UK. "It looks like if you tell someone something is important, it gets enhanced more."
Journal reference: Journal of Neuroscience, DOI: 10.1523/jneurosci.3575-10.2011