Getting a good night's sleep before an exam or a busy day at work has always been sound advice, and now new research helps to explain why sleep has such beneficial effects on our ability to recall information later. Research from the University of Exeter in the UK suggests that sleep has a two-fold way of improving our memory: it helps us to protect memories from being forgotten, while also making memories easier to access.

Using data sets published in previous memory research, experimental psychologist Nicholas Dumay looked at how effective people were in recalling made-up words. After initially being taught the words, participants were asked to recall them, and were tested again after 12 hours, during which they either slept or remained in a state of wakefulness.

The results of the study, published this month in the journal Cortex, suggest that after a period of sleep we are more likely to recall things that we could not actually remember while we were awake. Participants in the study showed greater recollection of the nonsense words after sleeping for 12 hours, whereas immediately after exposure they were not able to recall the terms as successfully.

The finding suggests that sleep can promote access to memory traces that prior to rest are too weak to be retrieved. Dumay found that this effect of sleep – helping to rescue unrecalled memories – is more pronounced than its other aid to our recollection: preventing the loss of memories that we can successfully recall prior to rest.

"Sleep almost doubles our chances of remembering previously unrecalled material," said Dumay in a statement. "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."

Dumay's findings add to a hugely active area of research investigating the ties between sleep and our mental processes. Recent studies have found that just one night of sleep loss can alter our genes and that poor sleep could ultimately be what triggers Alzheimer's memory loss.

Amid all the ongoing sleep research, however, the notion that frail memories can be sharpened and strengthened in our brains while we sleep remains to be explored, according to Dumay. The researcher believes the overnight memory boost occurs in the brain's hippocampus, which processes and replays encoded memories as we rest, causing people to effectively re-experience the day's events while they sleep.

"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," he said.