A good night’s sleep can be transformative. Among its benefits are improved energy and mood, better immune system functioning and blood sugar regulation, and greater alertness and ability to concentrate. Given all of these benefits, the fact that a third of the human lifespan is spent sleeping makes evolutionary sense. However, sleep appears to have another important function: helping us learn. Across a plethora of memory tasks—involving word lists, maze locations, auditory tones, and more—going to sleep after training yields better performance than remaining awake. This has prompted many sleep researchers to reach a provocative conclusion: beyond merely supporting learning, sleep is vital, and perhaps even directly responsible, for learning itself. 
 
 Recent discoveries from neuroscience provide insights into that possibility. Sleep appears to be important for long-term potentiation, a strengthening of signals between neurons that is widely regarded as a mechanism of learning and memory. Certain memories acquired during the day appear to be reactivated and “replayed” in the brain during sleep, which may help make them longer lasting. In some instances the amount of improvement that occurs on memory tasks positively correlates with the length of time spent in certain stages of sleep. These and other findings are generating great excitement among sleep researchers, as well as prompting heated debates about the degree to which sleep may or may not be involved in learning. 
 
 To date, most sleep and learning research has focused on recall, which is the capacity to remember information. However, new research by Stéphanie Mazza and colleagues at the University of Lyon, recently published in the journal Psychological Science, suggests another potential benefit of sleep: improved relearning. Relearning refers to the process of re-acquiring forgotten information. Because we cannot possibly remember all of the information that we encounter, it is often necessary to go back and learn that information again. That’s when relearning occurs—such as when preparing for an exam, taking a refresher course, or simply revisiting a topic after an extended period of time. According to this new research, sleep supercharges relearning: it can enable us to relearn twice as quickly and up to three times more effectively. 
 
 In the study, 40 French-speaking adult participants learned a list of 16 Swahili-French word pairs (for example, nyanya-tomate), relearned those word pairs after a period of 12 hours that did or did not include sleep, and were tested on their memory for the word pairs after one week and after six months. Each participant completed the study as part of a randomly-assigned “wake” or “sleep” group. In the wake group, the initial learning session occurred at 9 AM and the relearning session occurred at 9 PM on the same day; in the sleep group, the initial learning session occurred at 9 PM and the relearning session occurred at 9 AM on the next day, after a night of sleep. During the learning session, participants first studied all of the word pairs. Next, the Swahili word from each pair was shown by itself while participants attempted to recall its French equivalent. After each attempt, the correct answer was displayed, allowing participants to study further. The learning session concluded after each pair was successfully recalled once. At the relearning session, participants again attempted to recall the French word of each pair, and then continued cycling through the entire list until they could recall all pairs perfectly. Thus, by the time the relearning session was complete, any word pair that had been forgotten in the intervening 12 hours had been practiced until it could be correctly recalled once more. 
 
 During the initial learning session there were no observed performance differences between the sleep and wake groups, which suggests that a similar degree of learning occurred in both groups. However, substantial differences emerged at the relearning session: participants in the sleep group recalled an average of 10 word pairs on their first attempt, whereas those in the wake group could only muster up about 7; in addition, those that had slept took only about three cycles through the list to finish relearning, whereas those that had not slept needed twice that amount. Even the lowest-performing members of the sleep group—those that had forgotten more word pairs than other members and had the most to relearn—still took fewer cycles to relearn than the best performing (and least forgetful) members of the wake group. 
 
 Along with that impressive result, a benefit of having slept between the learning and relearning sessions was also evident over the long term. After one week, participants in the sleep group correctly recalled more word pairs than those in the wake group (an average of 15 vs. 11), and after six months, their recall performance, while attenuated due to some forgetting, was three times better than that of the wake group (an average of 9 vs. 3). Thus, despite the fact that both groups had practiced to a level of perfect recall during the relearning session, only those that had slept prior to relearning accrued any long-term memory benefits. 
 
 The fact that participants in the sleep group engaged in relearning was also critical for their improved long-term performance. In a follow-up to the main experiment, a control group of 20 additional participants followed nearly all of the same procedures as the sleep group, including training at the same times of day and sleeping after initial learning. However, in place of the relearning session, these participants only briefly practiced the list of word pairs one time. Without relearning, their subsequent recall performance was no better than the wake group. Thus, it was the combination of sleeping after initial learning, plus engaging in relearning itself, that generated long-term memory benefits. 
 
 Mazza and her colleagues interpreted their results as evidence that sleep “transforms” memories, making the effects of initial learning stronger and empowering subsequent relearning. By this interpretation, sleep not only prevents memories from being forgotten, it also makes it easier to restore memories during relearning. In support of this conclusion, the authors noted that total sleep time was positively correlated with recall and relearning performance (the longer participants slept, the better they tended to do on both). This pattern, similar to that observed in prior research, is consistent with two possibilities: either a sleep-specific mechanism boosts learning, which sets the stage for improved relearning, or sleep allows regularly occurring learning processes to occur without interference, which also boosts memory and expedites relearning. Both possibilities remain viable in this and related research, thus inviting future studies to tease the two apart. 
 
 Moreover, while this latest study reveals an important and largely unknown potential benefit of sleep, it remains to be determined whether the same benefits will manifest in other circumstances (such as when relearning non-verbal materials). It is also important to note that Mazza and her colleagues had their sleep and wake groups train at different times of day. In a recent meta-analysis of sleep and motor learning research, Tim Rickard and I found that such “varied time” experimental designs often yield better performance in sleep vs. wake groups regardless of whether there is a sleep-specific learning benefit. This may be due to the fact that humans vary in fatigue levels, alertness, and other characteristics according to the time of day and their natural urge to sleep—differences that may not necessarily manifest in the sleepiness surveys that participants of sleep studies are often asked to take (as occurred in the present study). Further research in which the sleep and wake groups are not subjected to such widely divergent schedules is needed to address this issue. 
 
 At present, these new results suggest that sleeping between study sessions can be a particularly potent learning strategy, and especially when learners take the time to repeatedly revisit and relearn in order to achieve a high level of performance. For instance, if a set of materials needs to be completely mastered, such as an official manual in preparation for a licensure exam, then those materials should be repeatedly studied across multiple study sessions and in between periods of sleep. Scheduling those study sessions to occur shortly before going to bed is also likely to improve future remembering and relearning. In a world where an uninterrupted night of sleep seems to be increasingly rare—and many of us try expensive and unproven ways to improve learning and memory—these may be some of the most compelling reasons yet to embrace the Land of Nod. ||||| Abstract 
 
 Both repeated practice and sleep improve long-term retention of information. The assumed common mechanism underlying these effects is memory reactivation, either on-line and effortful or off-line and effortless. In the study reported here, we investigated whether sleep-dependent memory consolidation could help to save practice time during relearning. During two sessions occurring 12 hr apart, 40 participants practiced foreign vocabulary until they reached a perfect level of performance. Half of them learned in the morning and relearned in the evening of a single day. The other half learned in the evening of one day, slept, and then relearned in the morning of the next day. Their retention was assessed 1 week later and 6 months later. We found that interleaving sleep between learning sessions not only reduced the amount of practice needed by half but also ensured much better long-term retention. Sleeping after learning is definitely a good strategy, but sleeping between two learning sessions is a better strategy. ||||| Although many of us can’t increase the working hours in the day, we can measurably increase our energy. Science supplies a useful way to understand the forces at play here. Physicists understand energy as the capacity to do work. Like time, energy is finite; but unlike time, it is renewable. Taking more time off is counterintuitive for most of us. The idea is also at odds with the prevailing work ethic in most companies, where downtime is typically viewed as time wasted. More than one-third of employees, for example, eat lunch at their desks on a regular basis. More than 50 percent assume they’ll work during their vacations. 
 
 In most workplaces, rewards still accrue to those who push the hardest and most continuously over time. But that doesn’t mean they’re the most productive. 
 
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 Spending more hours at work often leads to less time for sleep and insufficient sleep takes a substantial toll on performance. In a study of nearly 400 employees, published last year, researchers found that sleeping too little — defined as less than six hours each night — was one of the best predictors of on-the-job burn-out. A recent Harvard study estimated that sleep deprivation costs American companies $63.2 billion a year in lost productivity. 
 
 The Stanford researcher Cheri D. Mah found that when she got male basketball players to sleep 10 hours a night, their performances in practice dramatically improved: free-throw and three-point shooting each increased by an average of 9 percent. 
 
 Daytime naps have a similar effect on performance. When night shift air traffic controllers were given 40 minutes to nap — and slept an average of 19 minutes — they performed much better on tests that measured vigilance and reaction time. 
 
 Longer naps have an even more profound impact than shorter ones. Sara C. Mednick, a sleep researcher at the University of California, Riverside, found that a 60- to 90-minute nap improved memory test results as fully as did eight hours of sleep. 
 
 MORE vacations are similarly beneficial. In 2006, the accounting firm Ernst & Young did an internal study of its employees and found that for each additional 10 hours of vacation employees took, their year-end performance ratings from supervisors (on a scale of one to five) improved by 8 percent. Frequent vacationers were also significantly less likely to leave the firm. 
 
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 As athletes understand especially well, the greater the performance demand, the greater the need for renewal. When we’re under pressure, however, most of us experience the opposite impulse: to push harder rather than rest. This may explain why a recent survey by Harris Interactive found that Americans left an average of 9.2 vacation days unused in 2012 — up from 6.2 days in 2011. 
 
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 The importance of restoration is rooted in our physiology. Human beings aren’t designed to expend energy continuously. Rather, we’re meant to pulse between spending and recovering energy. 
 
 In the 1950s, the researchers William Dement and Nathaniel Kleitman discovered that we sleep in cycles of roughly 90 minutes, moving from light to deep sleep and back out again. They named this pattern the Basic-Rest Activity Cycle or BRAC. A decade later, Professor Kleitman discovered that this cycle recapitulates itself during our waking lives. 
 
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 The difference is that during the day we move from a state of alertness progressively into physiological fatigue approximately every 90 minutes. Our bodies regularly tell us to take a break, but we often override these signals and instead stoke ourselves up with caffeine, sugar and our own emergency reserves — the stress hormones adrenaline, noradrenaline and cortisol . 
 
 Working in 90-minute intervals turns out to be a prescription for maximizing productivity. Professor K. Anders Ericsson and his colleagues at Florida State University have studied elite performers, including musicians, athletes, actors and chess players. In each of these fields, Dr. Ericsson found that the best performers typically practice in uninterrupted sessions that last no more than 90 minutes. They begin in the morning, take a break between sessions, and rarely work for more than four and a half hours in any given day. 
 
 “To maximize gains from long-term practice,” Dr. Ericsson concluded, “individuals must avoid exhaustion and must limit practice to an amount from which they can completely recover on a daily or weekly basis.” 
 
 I’ve systematically built these principles into the way I write. For my first three books, I sat at my desk for up 10 hours a day. Each of the books took me at least a year to write. For my two most recent books, I wrote in three uninterrupted 90-minute sessions — beginning first thing in the morning, when my energy was highest — and took a break after each one. 
 
 Along the way, I learned that it’s not how long, but how well, you renew that matters most in terms of performance. Even renewal requires practice. The more rapidly and deeply I learned to quiet my mind and relax my body, the more restored I felt afterward. For one of the breaks, I ran. This generated mental and emotional renewal, but also turned out to be a time in which some of my best ideas came to me, unbidden. Writing just four and half hours a day, I completed both books in less than six months and spent my afternoons on less demanding work. 
 
 The power of renewal was so compelling to me that I’ve created a business around it that helps a range of companies including Google , Coca-Cola , Green Mountain Coffee , the Los Angeles Police Department, Cleveland Clinic and Genentech . 
 
 Our own offices are a laboratory for the principles we teach. Renewal is central to how we work. We dedicated space to a “renewal” room in which employees can nap, meditate or relax. We have a spacious lounge where employees hang out together and snack on healthy foods we provide. We encourage workers to take renewal breaks throughout the day, and to leave the office for lunch, which we often do together. We allow people to work from home several days a week, in part so they can avoid debilitating rush-hour commutes. Our workdays end at 6 p.m. and we don’t expect anyone to answer e-mail in the evenings or on the weekends. Employees receive four weeks of vacation from their first year. 
 
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 Our basic idea is that the energy employees bring to their jobs is far more important in terms of the value of their work than is the number of hours they work. By managing energy more skillfully, it’s possible to get more done, in less time, more sustainably. In a decade, no one has ever chosen to leave the company. Our secret is simple — and generally applicable. When we’re renewing, we’re truly renewing, so when we’re working, we can really work.
– It's well established through previous research that sleep after learning is best for many memory-related tasks, including word lists, mazes, auditory tones, and so on. Sleep seems so vital to recall that some speculate it is directly responsible for, not just supportive of, learning, reports Scientific American. So researchers out of the University of Lyon chose to investigate another aspect of learning—not recall but relearning, where something previously learned has been forgotten and must be re-acquired. Reporting in the journal Psychological Science in August, they wroite that, coupled with practice, "Sleep makes perfect," and that "sleeping after learning is definitely a good strategy, but sleeping between two learning sessions is a better strategy." The team tested 40 French-speaking adults tasked with learning 16 Swahili words. Some learned them at night, slept, and relearned them in the morning, while others learned the words in the morning, didn't sleep, and relearned them at night. The group that slept between sessions performed so much better that even those who forgot the most words relearned them faster during their morning session than the least forgetful members of the group relearning the words at night. The results don't clarify which is at play—sleep boosting learning, which helps improve later relearning, or sleep simply allowing for uninterrupted learning processes to speed up relearning—but either way, sleeping between studying seems worth a shot. (This woman only sleeps three hours a night.)