Sleep almost doubles the chances of remembering previously forgotten information, scientists found.
They believe sleep makes memories more accessible and sharpens our power of recall.
Volunteers taking part in the study were asked to remember made-up words either before a night's sleep or after 12 hours of wakefulness.
The "sleepers" were much better at recalling the words than the participants who remained awake.
Psychologist Dr Nicolas Dumay, from the University of Exeter, said: "Sleep almost doubles our chances of remembering previously unrecalled material.
"The post-sleep boost in memory accessibility may indicate that some memories are sharpened overnight.
"This supports the notion that, while asleep, we actively rehearse information flagged as important.
"More research is needed into the functional significance of this rehearsal and whether, for instance, it allows memories to be accessible in a wider range of contexts, hence making them more useful."
Dr Dumay believes the memory boost comes from the hippocampus, a part of the brain that plays a key role in recall.
During sleep, recently encoded memory episodes are "unzipped" and replayed to the region of the brain originally involved in their capture.
