Tuesday March 24 2015
Napping may help to consolidate memories
"A 45-minute power nap can boost your memory five-fold," reports The Independent.
This headline is based on a study that looked at the impact of napping on healthy volunteers’ ability to remember single words or word pairs in a memory test.
After being shown the words for the first time and then being tested on them, volunteers were split into two groups. The first group was allowed a 90-minute nap and the second group were made to stay awake.
It found that those who had a nap remembered similar numbers of word pairs after their nap as they had before their nap, while those who stayed awake tended to not remember as many.
The students tended to forget some of the single words between the two tests, regardless of whether they had a nap.
There are a number of limitations to this study – particularly its small size, with just 41 participants being analysed. This may be why the researchers were not quite able to rule out the idea that the differences between the groups occurred by chance. The limitations mean that we can’t conclusively say that napping is better for memory than not napping based on this study, particularly in real-world situations.
Sleep is known to be important for memory, and there is increasing interest in the effects of napping. For example, a study we discussed earlier this year suggested that napping improves memory retention in infants.
Where did the story come from?
The study was carried out by researchers from Saarland University in Germany. Funding was provided by the German Research Foundation.
The study was published in the peer-reviewed journal Neurobiology of Learning and Memory.
The UK media tended to overplay the findings of this small study. Most of them refer to a "five-fold" improvement in memory, which appears to come from a quote from one of the study authors. The author is also quoted as saying that: "A short nap at the office or in school is enough to significantly improve learning success."
This five-fold figure does not appear to be specifically mentioned in the research paper, and the differences between the groups at the end of the study were not quite large enough to rule out the idea that they occurred by chance.
Although the headlines talk about memory "improvement", what actually happened was that performance on the memory test stayed about the same after a nap, but got worse without one. We also can’t be sure whether the simple tests used in this study are representative of routine office or school tasks.
What kind of research was this?
This was a randomised controlled trial (RCT) looking at the effect of a nap on specific aspects of memory.
Sleep is thought to be important for "consolidating" our memories – essentially strengthening them and making it more likely that we remember. The researchers reported that a number of studies have shown that people perform better in certain memory tasks after sleeping than after staying awake for a similar period. However, they say that the effect of naps on different aspects of memory has been studied to a lesser degree.
The researchers wanted to look at the impact of naps on "associative memory" – the ability to learn and remember the relationship between two items – such as a person’s name, which relies on a part of the brain called the hippocampus. They also assessed "item memory" – the ability to remember whether we have seen or heard things before – which does not rely on the hippocampus.
An RCT is the best way to compare the effects of different treatments or interventions – in this case, a nap and a control (watching a DVD). This is because the groups being compared should be well balanced in terms of their characteristics, meaning only the intervention should differ between them and therefore be responsible for the differences in outcome. However, in small studies such as this, even randomly assigning people may not be able to achieve balanced groups.
What did the research involve?
The researchers enrolled healthy young university students and tested their memory for word pairs or single words they had been shown. They then randomly allocated them to either have up to a 90-minute nap and then watch a 30-minute DVD, or just watch DVDs for two hours. After this, they tested their memories for the words again, and compared the performance of those who napped and those who stayed awake.
There were 73 students who agreed to take part in the study, but 17 were excluded because the results of their initial memory test suggested that they were just guessing. An additional 15 were excluded after the test, as they performed particularly badly or they had not napped when they were meant to, or napped when they were not meant to. None of the students had sleep disorders or neurological problems, and they were all paid to take part in the study.
The memory test involved showing the students 120 unrelated word pairs (for the associative memory test) and 90 single words (for the item memory test), each appearing briefly on a screen, and asking them to remember them. About half an hour later, the students were shown 60 single words and 60 word pairs, and asked if these were words or pairs they had seen before.
The students then had their nap or watched the DVDs, depending on which group they had been assigned to. The DVDs only had music and images, and not words. Those who had a nap had their brainwaves monitored. They also watched about 30 minutes of one of the DVDs after they woke up to give them a bit of time to get over any residual sleepiness. The groups then did the word test again, this time with 120 word pairs and 120 single words.
The researchers compared the performance of those who napped and those who didn’t, both before and after the nap. They also looked at whether brainwave activity during the nap predicted a person’s performance on the memory test.
What were the basic results?
The napping group slept for about 64 minutes, on average.
The researchers found that both those who napped and those who didn’t performed worse on their second single word (item) memory test than they had at the start of the study shortly after they first saw the words.
The group who did not nap also performed worse in their second word pair (associative) memory task than they had at the start of the study. However, those who had a nap performed similarly on the word pair memory task at the start of the study and after their nap. This suggested that the nap had helped them to retain their memories of the words. The difference between the groups in their performance on the second word pair test was near to, but not quite reaching, what would be considered statistically significant (that is, enough to have a high level of certainty that it did not occur by chance).
How did the researchers interpret the results?
The researchers concluded that "these results speak for a selective beneficial impact of naps on hippocampus-dependent memories".
This small study has suggested that in healthy adults, a nap of about an hour might help to retain one type of newly formed memory – associative memory of unrelated pairs of words – but not item memory of single words.
While the study’s random allocation of participants is a strength, there are limitations:
- The study was small and only included healthy young adults. The results may not apply to other groups of people, and ideally would be confirmed in larger studies.
- While the reduction in associative memory in the group that stayed awake was statistically significant, the difference between the napping and non-napping groups in the word pair test at the end of the study was almost, but not quite, large enough to reach this level. That is, it was not quite enough to give a high level of certainty that it did not occur by chance. This may be due to the relatively small size of the study, and again suggests that larger studies are needed.
- Some students were excluded after they had been randomly allocated to their groups; this can lead to imbalance between the groups and affect results. Ideally, results would have been shown both with and without those students included, to see if it made a difference. Analysing all the participants in the groups to which they were assigned, regardless of what happens to them, is an approach known as "intention to treat".
- We don’t know how long the effect of the nap would last, as participants were only assessed a short time after their nap – with the tests all happening on one day.
- The tests were simple word-based memory tests, and naps only affected one aspect of memory. We don’t know whether the naps might make a difference in remembering more complex information or different types of memory not tested in this study.
Overall, the study by itself does not conclusively show the benefits of naps on memory in our day-to-day lives.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.