Tired? Take A Nap

Jessica Payne and Matthew Walker
Harvard University and University of California at Berkeley
Sleep is such a fundamental biological drive that it's shared by practically every species, from fruit flies to humans. Indeed, sleep is so essential that animals will die as quickly from sleep deprivation as they will from food deprivation. And yet despite such alarming evidence (not to mention all the things your mother told you about the importance of sleep), human beings are one of the few species that deliberately deprive themselves of sleep. Why do we do this? Most would blame the frantic pace of modern life: we live in a 24/7 world, fuelled by ever-present deadlines, demands and responsibilities. With so much to do and so little time, it seems tempting, or perhaps even necessary, to shave off a few hours of sleep in order to get things done. Given this chronic shortage of shuteye, wouldn't it be wonderful if a quick nap could refresh us mentally and improve our memories? Results from a recent study by Lahl et al. (2008) suggest that a mere six-minute nap may be able do just that. Before discussing this intriguing finding, some background is in order. The past decade has witnessed a boom in reports demonstrating that sleep after learning is critical for the offline "consolidation" of memories. The process is what allows the brain to convert newly learned information into long-term storage in the brain. Sleep has been shown to promote consolidation of various kinds of memory, from procedural skills (for example, learning to play a musical instrument or mastering a sport), to episodic memories (such as remembering facts and experiences learned during the day). These benefits were initially thought to require a full night of sleep, but more recent studies suggest that, for episodic memories, a 60-to 90-minute nap may generate the same memory benefits in much less time. Until the study by Lahl et al., however, many scientists would have been skeptical about the efficacy of a nap that was any shorter. In part, this skepticism is because the biological mechanisms believed to be necessary for cementing long-term memories - particularly the synthesis of new proteins needed to strengthen synaptic connections - takes at least this long. Nonetheless, there are many anecdotal accounts describing the cognitive benefits of ultra-short "power naps." The study by Lahl et al. provides the first empirical support for such a claim. In this study, 18 college students participated in three different experimental conditions: taking a long nap (35 minutes), a short nap (six minutes) or remaining awake. Prior to napping (or remaining awake), participants learned a list of 30 words and were tested on these words one hour later. Learning always took place at 1 P.M. and testing took place at 2 P.M.; the question was whether napping would produce better memory retention than wakefulness. The short nap consisted of relatively light sleep, mostly stages 1 and 2 non-rapid eye movement (NREM) sleep. The long nap contained even more of this light sleep, along with about 10 minutes of deeper slow-wave sleep (stages 3 and 4 NREM). Similar to a previous study by Tucker et al. (2006), the long nap resulted in better memory for the words than wakefulness. But, remarkably, so did the short nap, albeit at a lower level. Nevertheless, this suggests that just six minutes of sleep can produce significant memory benefits. As interesting as these findings are, several questions remain. First, participants were tested after just one hour, leaving unclear whether the memory improvement persists long term (a defining characteristic of consolidation) or is simply transitory. Would a mere six minutes of afternoon sleep promote improved recall one day or even one week later? Second, it is important to keep in mind that the memory improvement seen by Lahl et al. is modest. The long nap increased recall above wakefulness by about two words, and the short nap by a little more than one word. This finding leaves open questions about the utility of such short sleep, relative to obtaining a full night. Perhaps most intriguing of all is the underlying mechanism that generates these effects. Is there something about the neurobiology of sleep that enhances memory, even in short bursts? Or is it the lack of deleterious waking mental activity, which might interfere with memory storage? Because no specific stage of sleep correlated with the memory benefit, we are left hunting for clues. It is hard to imagine any known biological processes operating quickly enough to create the new synaptic structures necessary for memory consolidation within such a short timeframe. However, one possible answer resides in the electrical signals of sleep. For example, short nappers obtained a few minutes of stage 2 sleep, a stage that produces "sleep spindles." These powerful synchronous bursts of electrical activity have been linked to memory stabilization. Alternatively, such short naps may not necessarily change the strength of the memory, but rather allow easier access to it. That is, sleep, and just a small amount, may "grease" the recall-shoot that memories slide down, allowing easier access to the material. Yet another idea is that ultra-short naps may rapidly refresh regions of the prefrontal lobe (which suffer the greatest burden when sleep deprived), and, as a result, allow the brain to more accurately search its storehouse of memories. Regardless of one's interpretation of the results, the experimental benefit of napping seems clear. This finding should come as good news for those of us who feel guilty or lazy for napping⎯an unfortunate reflection of a society out of touch with our basic biological needs. Nonetheless, we should still remember what our mothers might say: "a nap is no substitute for a good night's sleep." Maybe it is time we all woke up to this reality.