New research shows that a full night's rest recharges the brain and boosts memory, attention span-even creativity!

SARAH GROTH HAD only two more weeks before her trip to Israel, and if things kept going as they were, she was going to have trouble asking locals how to find the restroom. Although she stayed up late each night studying phrases, and got up early the next morning to cram some more, by the next day it always seemed she was back where she started--hardly past "shalom," let alone to "Kamah zeh o'leh?" ("How much does this cost?") . When Sarah, 25, told a psychology student friend about her problems, however, he surprised her. He didn't suggest a new way to lodge tourist phrases into her head or even recommend an iPhone app for it. Instead, he suggested she get more than the six hours of sleep she had been allowing herself while she was cramming for the trip.

It worked. Within a week, Sarah was back to her normal seven-and-a-half hours of sleep, and suddenly "Eifo ha'sherutim?" ("Where is the bathroom?") was rolling off her tongue. Sarah had unwittingly replicated, on a very small scale, a growing number of experiments showing that sleep is a powerful force for consolidating memories (making them stick), not to mention other cognitive functions as diverse as creative thinking and split-second decision making. "Attention, executive function, judgment-they're all impaired by a lack of sleep," says psychologist Howard Nusbaum of the University of Chicago.

Emerging research is finally shedding much-needed light on a longstanding mystery: Why do humans spend about one-third of our lives catching z's? Over the years, scientists have speculated that sleep might promote wound healing, conserve energy, recharge the immune system, or simply (no kidding!) be nature's way of preventing us from walking around after dark and getting eaten by lions. While most of those theories have fallen by the wayside, the idea that sleep promotes some aspects of physical health has held up; insufficient sleep can indeed cause cardiovascular disease, diabetes, and obesity (see sidebar on the next page). Given that sleep is a brain state, it makes sense that it affects the brain--and not just in causing it to generate the bizarre narratives we call dreams.

The new discoveries about the cognitive function of sleep make our nation's collective failure to get enough shut-eye even more alarming. In a poll released by the National Sleep Foundation in March, 43 percent of Americans between 13 and 64 said they rarely get a good night's sleep on weeknights (when they can't sleep late the next morning); 60 percent said they experience a sleep problem such as waking in the night every or almost every night. Most say they need about seven and a half hours of sleep to feel their best--in line with expert advice that adults get between seven and eight hours--but report getting, on average, six hours and 55 minutes. About 15 percent of adults under 65 say they sleep less than six hours on weeknights. Among the sleep-deprived who have jobs, 74 percent of those over 30 said drowsiness impairs their work. It's amazing more of us aren't using toothpicks to prop up our eyelids.

Many studies--not to mention personal experience--have shown that sleep impairs brain function in a way not too different from being slightly tipsy. In a 2009 study, psychologist Todd Maddox of the University of Texas and colleagues found that sleep acts even more specifically: It gives us the ability to make the kind of split-second responses that firefighters, police officers, physicians, and others often need to make. But when people are sleep deprived, as members of these professions often are, they shift from a split-second, gut-level process of categorizing information to a slower, rule-based process--the difference between going with your gut and trying to reason something out. When the decision comes in an area in which you are an expert, and where gut-level decisions are more accurate, this switch can have tragic consequences. "The fast and accurate strategy is critical in situations when soldiers need to make split-second decisions about whether a potential target is an enemy soldier, a civilian, or one of their own," says Maddox, whose study analyzed the effect of sleep deprivation on West Point cadets. Their accuracy at identifying a soldier as friend or foe was 78 percent after normal sleep but 71 percent after sleepdeprivation.

Dr. Chiara Cirelli has an idea about why that happens. Giving new meaning to the term "asleep on your feet," she and colleagues found that when rats are sleep-deprived. whole regions of the brain briefly go "off-line" exactly as they do during sleep--except that the rats are awake. "Brain regions that are used the most, such as those involved in learning a motor task, get the most tired and seem to go to sleep without waiting for the rest of the brain to," says Cirelli, who with her colleagues at the University of Wisconsin, Madison, reported the finding in April. Such "local sleep," she suggests, may explain the "cognitive impairments, including defective judgment and irritability" that occur in sleep-deprived humans who nevertheless "give an outward impression of wakefulness." That colleague of yours may seem awake, but whole neighborhoods of his brain are taking catnaps.

Perhaps the most serious consequence of inadequate sleep is not the impairments you suffer, but the benefits you don't get. For one, sleep helps the brain learn complicated tasks. When Nusbaum and colleagues trained volunteers to understand distorted, computer-synthesized speech in a 2003 study, participants improved from catching only one-in-five words at first to getting 40 percent. After being awake for a standard 12 hours, they had fallen back to understanding only 30 percent. They had forgotten a quarter of what they'd learned. After a good night's sleep, however, they were back to 40 percent. "By the end of the day it may seem as if you have forgotten a recently learned skill," says Nusbaum, "but sleep can bring it back."

That works for something more practical than trying to understand a speech-slurring computer. For a 2008 study, Nusbaum's team had volunteers practice Unreal Tournament 2003 and Quake 3, video games in which the player has to navigate through and remember complex environments, kill enemy robots, and avoid being killed--all by using the keyboard and mouse in complicated combinations. In the experiment, novice players got just under an hour of practice, earned a score, and were then tested again on their ability to kill robots and stay alive. Players who trained in the morning and stayed awake all day improved only 4.3 points by evening; after a good night's sleep they had improved by 9.8 points. Players who trained at night, then went to sleep and were retested in the morning did even better, up by lo points after a good night's rest. "Performance on newly learned skills degrades significantly during waking hours," says Nusbaum, "but sleep can protect you from forgetting and even restore information or learning that seems to have been lost."

At least two mechanisms might explain the ability of sleep to enhance learning and memory. One possibility is that, over the course of a day, "you have lots of experiences that clutter up your brain," says Nusbaum. "Sleep might sweep away the clutter, leaving behind only what's important." Indeed, Cirelli and colleagues have found that sleep seems to degrade some of the brain's synapses-connections allowing neurons (nerve cells) to communicate with one another. Losing synapses would seem like a bad idea, except for this: Maintaining synapses takes so much energy and space that if the brain kept them all it would soon be as dysfunctional as the most extreme hoarder. "Sleep is necessary for our brain to learn new things every day," says Cirelli. "Its sweeps the brain clean of the weakest synapses--those holding memories of the day's random experiences and others that we don't need. You need to clean out the junk filling your brain in order to learn more."

The other way sleep might enhance brain function is by strengthening synapses that hold important memories. One way this works is by replaying the patterns of neuronal firing in circuits that laid down the memory initially. If a set of neurons fired in a particular sequence when a rat learned to navigate a maze or when a bird learned a new song, then sleep replays that sequence, usually during "slow-wave" sleep. That replay strengthens the memory, sort of the way going over a faint signature makes it bolder and more durable. "The replay story is very well established, including in primates," says Cirelli.

In contrast, when we're awake the circuit that laid down a memory may become conscripted for other activities, which could reduce the stability of the new trace just as writing one name atop another makes both difficult to read. All this could explain why "sleeping on it" makes sense; if you snooze soon after you learn something rather than letting the day's clutter interfere, you're more likely to retain it. Indeed, a 2004 study found that the more complex a learned motor sequence (the sequence of physical motions required to complete an action), the greater the post-sleep improvements. You might just want to schedule those tennis lessons for the evening.

And tell your video-gaming kids or grandkids to save the playing until after homework. When novice gamers played Guitar Hero III: Legends of Rock, which requires precise finger motions to master, after some practice they got 61 percent of the notes right. When players stayed awake and tried the practiced songs hours later, their performance accuracy was essentially unchanged. But players who went to sleep after their session scored 68 percent the next morning--even better than when the sequence of notes was fresh in their minds.

As scientists have learned more and more about the benefits of sleep for learning, memory, decision-making, and judgment, an obvious question arises: Sleep does not just press a universal "save" key, so how does it "know" what to preserve? One clue is the fact that sleep selectively preserves memories that have the greatest emotional resonance while erasing neutral details. That's why you can remember what your future spouse said the first time you met but not what furniture was in the room. In one typical study, scientists found that when people saw upsetting photographs of a crashed car, those who stayed awake for the next 12 hours forgot almost everything. Those tested after a night of sleep recalled the smashed car as accurately as people tested just 30 minutes after seeing the images.

This suggests that, during sleep, the brain "unbinds" the individual components of a memory, separating wheat from chaff. "The brain seems to do some sort of calculation to select what we need to remember," says cognitive neuroscientist Jessica Payne of Notre Dame University. It tags three kinds of memory: emotional memories, "gist" memories--the general sense of what we have experienced, heard, or read--and memories the waking brain deemed important.

Unbinding memories may be a first step toward reorganizing them in a way that may spur novel thinking and creative ideas. Creativity, after all, arises from putting together pieces of what we know in fresh ways. "Sleep has a profound effect on our cognitive abilities," says Payne, who (taking her own research to heart) tries to sleep eight hours a night. "Memories don't simply get consolidated during sleep; they undergo dynamic processing. The brain somehow sorts through the different components of a memory, finding connections and regularities, which in turn helps us form novel ideas."

Skeptics who doubt that sleep is crucial often bring up dolphins, which are thought not to sleep because they never stop swimming. In fact, dolphins have mastered "unihemispheric sleep," in which one eye is closed and one half of the brain shows the slow waves characteristic of deep sleep, while the other eye and the other hemisphere are wide awake. "The fact that dolphins have this remarkable specialization shows that sleep must serve some essential function," Cirelli says. "It must be important, because all animals do it."

Sleep and Heart Disease

Dark circles under your eyes are the least you have to worry about if you get less sleep than the seven to eight hours a night that the American Academy of Sleep Medicine recommends for adults; inadequate sleep "can make you vulnerable to heart disease and stroke," says cardiologist Marc Gillinov of the Cleveland Clinic. When poor sleep is the result of sleep apnea--from which an estimated 4 percent of Americans suffer--interrupted oxygen intake causes abnormal heart rhythms, he explains, and struggling to breathe can cause blood pressure to spike as high as 240 mmHg, increasing the stroke risk.

Only now are scientists seeing how important.

Sleep and Diabetes

The most obvious way poor sleep can increase your risk of developing type-2 diabetes is by disrupting appetite-regulating hormones or turning you into a midnight snacker, causing you to pack on the pounds. But new research shows that poor or insufficient sleep can also impair your cells' insulin sensitivity, which allows glucose to build up in the blood, resulting in diabetes. Indeed, only three nights of disrupted or insufficient sleep increases insulin resistance, finds sleep specialist Eve Van Cauter of the University of Chicago.

Your chances of developing type-2 diabetes double if you get less than seven hours of sleep a night, scientists at Université Laval in Quebec City reported in a 2009 study in the journal Sleep Medicine; it triples if you get only five to six hours.

One reason for the sleep-diabetes connection is that getting too few hours of sleep or waking up at night disrupts slow-wave sleep, the deepest, nondreaming, restorative sleep stage. Slow-wave sleep coincides with hormonal changes that affect glucose regulation. When slow-wave sleep is disrupted, insulin sensitivity decreases, leading to increased diabetes risk.

Sleep and Obesity

It's not just midnight snacking; getting too little sleep can wreak havoc on the hormones that control appetite and metabolism. Evidence that inadequate sleep can increase the risk of obesity began emerging a decade ago and has gotten stronger with time. At first the link was only circumstantial; as the amount of sleep American adults were getting fell from 8.5 hours in 1960 to just under 7 hours in 2008, obesity rose from 13 percent in 1960 to 34 percent today. In addition, 30 out of 35 studies in adults have found an association between inadequate sleep and being overweight, as have 20 out of 20 in children or adolescents.

This kind of research can't prove inadequate sleep is causing obesity rather than the other way around; it could well be that being overweight causes poor sleep. But rigorous lab experiments as well as studies following people for several years have teased out what is cause and what is effect, and inadequate sleep is looking guilty as charged.

The evidence is certainly mounting that inadequate sleep is a likely contributor to the nation's troubling obesity epidemic.

Drowsiness can impair work and cut down on reaction time.

Sleep apnea can interrupt oxygen intake, resulting in high blood pressure and abnormal heart rhythms.

A video game study revealed that the rate of improvement dramatically increased after a good night's sleep.

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By Sharon Begley