Tuesday September 24 2013
Daytime napping may help the brain store memories
“’Afternoon naps' aid children's learning,” BBC News reports. A new study has found that toddlers who had Spanish-style siestas performed better in learning tasks compared to children who stayed awake.
This headline is based on a small study from the US which examined the effect of a midday nap on children’s ability to recall the location of pictures on a grid, which they had learned that morning when playing a memory game.
The study found that children were better able to recall the location of the pictures later in the day if they had taken a nap in the early afternoon, compared to staying awake throughout the day. Memory was also better the next morning, which the researchers suggest means that the benefits of a daytime nap cannot be made up for with overnight sleep.
The researchers speculate that this improvement may be due to what is known as a sleep spindle. This a burst of brain activity that occurs during sleep which may help the brain ‘integrate’ recent events into the long-term memory (though this hypothesis remains unproven).
Limitations of the study include its small size and the fact that it examined only one type of memory ability (declarative memory, which is the ability to recall previously learned knowledge, such as the nine times table).
With these limitations in mind, the results are intriguing and suggest napping may benefit children in ways that move beyond its impact on attention and afternoon sleepiness.
Where did the story come from?
The study was carried out by researchers from the University of Amherst in the US and was funded by the US National Institutes of Health and a research grant from the University’s Commonwealth College.
The study was published in the peer-reviewed journal Proceedings of the National Academy of Sciences (PNAS). PNAS is an open access journal so the study is free to read online or download (PDF, 661Kb).
Both BBC News and The Guardian covered the research appropriately, including an emphasis on the small study size.
What kind of research was this?
This was a cross-over study that assessed the impact of an afternoon nap on the memory of preschool children. (These types of study are usually randomised but this was not the case with this study).
Researchers hypothesised that daytime napping plays a role in early childhood memory by allowing information gathered during waking hours to be consolidated (improving the efficiency of recalling stored information) during short sleeps.
To determine possible mechanisms by which afternoon naps may exert an effect on memory, the researchers conducted a small laboratory based study that examined brain activity while the pre-schoolers slept. They determined that a measure of brain activity during sleep, known as sleep spindle density, was associated with recall.
What did the research involve?
The research included 77 preschool children between the ages of 36 and 67 months. Overall, 40 children were included in the analysis. The children completed a visuospatial task (or less technically, they played a memory game) in the morning at 10:00am.
The task/game involved learning the position of 9 to 12 pictures displayed in a grid on a screen. The pictures were hidden, one picture at a time was displayed on the right side of the screen and the children were asked to locate the same picture in the grid and feedback was provided. This encoding/playing was continued until the children had successfully identified 75% of the pictures.
Finally, the same memory task was repeated (pictures were hidden, identical pictures were displayed, children tried to recall where the matching item was in the grid), this time without feedback, and the children’s ability to recall picture location was assessed – this served as the baseline measurement.
Later that day, between 1:00pm and 3:00pm, half of the children took a nap and half stayed awake. All children then completed the task/game that afternoon at 3:30pm (delayed recall) and again the next morning at 10:00am (24 hours recall).
Each child completed both sequences (one day they napped, another day they stayed awake), and ability to remember picture location was compared between the two sequences.
The researchers also assessed child-reported sleepiness and experimenter-rated sleepiness of the children in the afternoons. This was done in order to assess whether differences in performance on the tests were due to naps reducing fatigue or increasing attention, rather than memory consolidation during sleep as hypothesised.
They also examined regularity of child napping, as reported by parents, to see if the effect differed depending on child sleeping habits.
What were the basic results?
On average, the children spent 78 minutes napping when they were included in the nap sequence. Performance on the memory test was similar between the two groups at baseline.
Performance on the delayed recall measurement (at 3:30pm) and the 24 hour recall were significantly better when the children had napped than when they had stayed awake:
- baseline recall accuracy, nap vs. no nap (approximately 76% vs. 75%,)
- delayed recall accuracy, nap vs. no nap (approximately 77% vs. 64%)
- 24 hour recall accuracy, nap vs. no nap (approximately 78% vs. 63%)
The researchers also found no significant differences in child-reported sleepiness in the nap vs. no nap conditions. When looking at the experimenter-rated measures, they found that child sleepiness was greater following the nap compared to the non-napping sequence.
Further analysis found a difference in effect when analysis was stratified according to regularity of napping. The positive effect on memory of the two hour preschool-based nap was greatest amongst the 17 children whose parents reported that the child napped five or more days each week, while the 10 children who napped on fewer than two days each week saw no benefit.
How did the researchers interpret the results?
The researchers conclude that an early afternoon nap is clearly beneficial in terms of memory retention amongst preschool children, and that the negative effects of missing daytime naps cannot be made up during night time sleep.
They highlighted the fact that there was a lack of difference in child-rated sleepiness, and an increase in experimenter rated sleepiness after naps.
There were also significant differences between the groups in performance on the 24 hour recall test (conducted after a night’s sleep). All of the points, they concluded, indicate that the differences in memory are due to processes during the nap as opposed to indirectly due to its impact on fatigue and attention.
This small study suggests that afternoon naps may have benefits in terms of the visual memory of preschool students.
Though there is some uncertainty about the ‘direction’ of the effects that were assessed by the researchers. It could be the case that a decline in memory recall ability in regular nappers was due to them being ‘deprived’ of their usual afternoon nap, as opposed to an increase in recall when additional naps are introduced.
That is, children who napped five or more times a week saw reductions in recall when they did not nap. While children who napped less than twice a week saw less decline in recall ability when kept awake during the early afternoon.
One key limitation of the study is the inclusion in the analysis of children who completed both the nap and wake conditions. Of the 77 children recruited into the study, 48% were excluded from the analysis because they were unable to complete either the napping or wake condition, or failed to complete the memory task, or because their immediate recall (the baseline measurement) was 100%. This may have introduced selection bias into the study as the children included in the final analysis may not be truly representative of their peers.
The authors’ conclusions on the unique process-based benefits of sleep are supported in part by measures of sleepiness as reported by experimenters. However, it is unclear if the experimenters were blinded to whether or not the child had napped during the afternoon; lack of blinding may have biased the results. Additionally, these measures were not reported to have been included as part of the statistical analysis, so it is unclear if significant differences were found based on child fatigue.
The researchers suggest that the results of their sleep laboratory sub-study suggest that benefits are derived due to processes unique to sleep. However, this portion of the study specifically recruited children based on their likelihood to sleep in a lab setting, and thus included mainly habitual nappers. Whether the findings apply to children who nap infrequently is unclear based on this study.
This small study assessed the impact of daytime naps on one specific type of memory. So this cannot be interpreted to mean that napping improves child memory across the board.
The researchers suggest that their findings should be considered when making decisions about whether or not to include an early afternoon sleep session in the nursery or pre-school schedule.
There are no official guidelines about daytime napping, but the Millpond Children’s Sleep Clinic (an international private sleep clinic specialising in child sleep problems), recommends that toddlers get around one hour of sleep during the day. Once a child reaches the age of four they then do not usually require regular afternoon naps.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.