Sleep May Unlock Previously Unavailable Memories

Sleep and memory. Their relationship is complex, but researchers have established that getting a good night's rest is instrumental in keeping forgetfulness at bay, in part because, when sleeping, your brain "replays" activity from earlier in the day.

A new study published in the journal Cortex attempts to further parse this relationship by asking: Is it possible that sleep doesn't simply protect memories from fading away, but actually makes them easier to retrieve? If the theory is correct, sleep doesn't just sharpen existing memory – it unlocks previously inaccessible ones.

To test the question, Nicolas Dumay, a professor at the University of Exeter, conducted an experiment in which 72 subjects were tasked with learning 24 made-up words (such as frenzilk) over the course of 45 minutes. Immediately afterward, to test their recall, the participants had three minutes to record as many words as they could remember from the list.

Next, the group was divided in two; half of the subjects remained awake for 12 hours, while the other half got a night's sleep. At the end of the 12-hour period, both groups were retested to see how many fictitious words they could remember.

Overall, participants who had slept outperformed those that hadn't. On average, they remembered 68 percent of the fictitious words that they'd recalled on the first test. Those who hadn't slept only remembered 46 percent of them, which echoes what previous research have already established: Sleep is a memory aid.

Dumay was more interested in 'gained' words, i.e. words that participants hadn't been able to recall in the first test, but somehow remembered when they were retested 12 hours later. Here, the difference between the two groups' performances was starker. Participants who hadn't slept recalled about 7 percent of the made-up words that they had not remembered when first tested; for participants who had slept, that percentage jumped to 17 percent.

This, of course, is one relatively small study, but according to Dumay, it suggests that sleep allows the brain to unlock weak or shaky memories. While further research is needed to determine why this happens, he posits that as we sleep, recent memories are sped up and replayed in the hippocampus, which results "in the subject re-experiencing them [and] leads to strengthening and increased accessibility."

If reproducable, the study has implications beyond education. "Although in this study, the material to be learnt was linguistic, the findings should be particularly useful to those interviewing victims or witnesses of crime scenes," Dumay says.

If you take away one practical tip from all of this, let it be this: Sleep is memory's friend. Says Dumay, "studying the morning of an exam may not be such a great idea!"

So the next time you are cramming for a test, a presentation, or anything else that requires the retention of information, make sure you set aside time to rest beforehand to avoid the terrible feeling of reaching for something you thought you knew only to find that somehow, miraculously and inconveniently, it’s gone.