The association between sleep and memory performance has been relatively well-documented. Studies in animals and humans have shown memory deficits in both declarative and procedural memory associated with sleep deprivation. (The subject is slightly complicated because whether sleep deprivation affects a task can depend on particular task demands and may vary from study to study. See this review for more details.)
What has received much less examination is the role of daytime sleep in aiding memory performance. Only a few studies have examined whether the “power nap” aids retention, and they have produced sometimes contradictory results. (For these previous studies, see here, here, and here.) The results of these earlier studies differ over whether procedural or declarative memory is improved during daytime naps or whether the “napping effect” was related to time spent in slow wave sleep (SWS) — a component of non-REM sleep otherwise known as deep sleep or delta sleep — as opposed total sleep time (TST).
Lahl et al. publishing in the Journal of Sleep Research attempt to address these discrepancies by testing the memory recall of subjects after short naps during the day.
Lahl et al. took subjects and gave them a list of 30 adjectives to memorize over 2 minutes. Then, they would let a napping group sleep for different specified periods — from 5 min to about an hour — while the control group would stay awake for that hour. When subjects were awake they would play a video game that did not include verbal components to make sure that they were not rehearsing the words during the interim. At the end of the hour, the subjects were given unlimited time to recall as many of the 30 adjectives as they could.
Here are the results:
The short nap condition includes subjects that were woken up after approximately 5 minutes of sleep. (The authors state that this group is “not longer than 6 minutes on average.”) The long nap condition subjects that were woken up after approximately 50 minutes of sleep. (I imagine that the “approximate” nature of these measures is because people fall asleep and wake up at different speeds.)
As you can see, there is an improvement over the waking condition for even very short naps on the order of 6 minutes.
The authors bring up an interesting issue in interpreting this data. If you look within groups for variations in performance, you would expect that the individuals who slept the longest should do the best. For example, in the long nap group you would expect people who napped 50 minutes to do better than those who napped only 40 minutes. (Once again, the variability in time napped is because people fall asleep at different rates.) However, when the authors do this, they found that no such a correlation exists. Yet, there is still an increase in performance between the short nap and long nap groups. These results conflict because they suggest in one case that sleep duration doesn’t matter and in the other case that it does.
The authors resolve this issue by suggesting that there might be a stage-specific benefit in sleep. For example, if one critical factor in improving performance happens when sleep is initiated, this would explain the rapid improvement in performance for the short nappers. Likewise, perhaps some time between 5 and 50 minutes, the nappers enter another sleep stage that also confers benefits on performance.
All of this plays into a continuing controversy in the sleep world about what sleep is actually doing to aid in memory:
In view of the growing body of results demonstrating sleep-related memory improvements, today probably only few researchers would still claim that there is absolutely no relationship between sleep and memory and the study of daytime naps provides yet another facet within this converging line of evidence. What is much less clear is the question of whether this relationship is of any significance regarding the function(s) of sleep, i.e. whether the role of sleep in memory consolidation is a passive or a functionally active one. Interference theory is the main theoretical framework for a passive role of sleep holding that sleep simply shelters memories from retrograde interference. For the interference account, it is hard to explain why a very brief sleep episode should provide a significant effect on memory since the only critical factor is the length of the sleep period during which the brain is protected from new information input. Consolidation theory on the other hand proposes an active or functional role of sleep in memory consolidation by triggering active processes of encoding during sleep (onset). If it was possible to confirm the effect of strengthening memories during sleep initialization in future research this would provide a strong argument for the consolidation account and therefore for the idea that memory consolidation is actually a functional target of sleep. (Citations removed. Emphasis mine.)
It appears to me that this study supports the consolidation account of sleep i.e. that particular stages of sleep (such as sleep onset) initiate active processes of consolidation that improve later memory performance. On the other hand, the authors did not find that performance was related to the amount of NREM sleep as opposed to REM sleep, so it is difficult to figure out (at least from this data) which is the critical stage in sleep for improving performance in this task. Clearly continuing experiments with both daytime and nighttime sleepers will be necessary to tack down which sleep stages help which types of tasks.
All-in-all though I think this study is a powerful vindication of the power nap. For at least some cognitive tasks even a tiny, tiny sleep can make you do better.
I do have one methodological question though. The authors mention that all the subjects in this study were university students. In my experience, university students are chronically sleep-deprived. Do you think that forms a confound? Would naps be helpful for the non-sleep-deprived?
Hat-tip: Faculty of 1000
LAHL, O., WISPEL, C., WILLIGENS, B., PIETROWSKY, R. (2008). An ultra short episode of sleep is sufficient to promote declarative memory performance. Journal of Sleep Research, 17(1), 3-10. DOI: 10.1111/j.1365-2869.2008.00622.x