Getting adequate sleep improves one's ability to remember and helps the brain maintain a positive emotional outlook, make better judgments and arrive at creative insights, sleep researchers say.
And studies show that pulling an “all-nighter” to study for an exam is counterproductive. Getting a good night's sleep between learning a task and being tested on it improves learning, says Robert Stickgold, an associate professor of psychiatry at Harvard Medical School. A night's sleep taken after memorizing a list of words, for example, not only solidifies the memorized material but “keeps it from being driven out by other information,” he says.
Researchers don't yet know for sure how the different stages of sleep affect memory, but they do know that sleep improves mental functioning on tasks that go well beyond simple memorization skills.
“Probably the sexiest thing we do with our brains is pick out the important stuff” from the bombardment of input we receive each day, says Stickgold. Sleep aids in that undertaking.
In experiments people went to sleep after viewing pictures in which a strong negative image appears against a background of more neutral images. Upon awakening “you forget everything else, but you remember the emotional objects,” Stickgold says, indicating that the sleeping brain strengthens memories that matter to us most strongly.
Sleep also helps us figure out rules and principles that connect information, enabling us to have insights, says Stickgold. In one test people are asked to perform “a tedious math task” that can be solved by “a sneaky shortcut” that they aren't told about but that can be figured out. Only one-quarter of the subjects figure out the shortcut at first, he says, but two-thirds will figure it out if they've had a night's sleep before they're asked about it.
Most everyday learning, however, has no simple cause-and-effect relationship between an event and a following event. Thus, many of our judgments are based on “probabilistic” reasoning — in which we use our accumulated experience to judge the odds of what will happen. For example, we may decide whether to take an umbrella after a glance out the window or whether or not to share a thought with a friend based on his or her apparent mood at the time.
“Probabilistic” rules are “stored in the brain in a manner I can't even imagine,” Stickgold says, but sleep definitely helps the brain sort through our experience to develop the rules.
In one scientific trial of probabilistic learning, a person is dealt cards from a playing deck, with a certain event occurring after a deal of hearts 100 percent of the time, after a deal of clubs 80 percent of the time, and so on. As the cards are dealt randomly, the subject is asked after each card whether the event will occur and gets immediate feedback about whether the guess was correct. When tested on whether they'd correctly internalized the probability rules, after a good night's sleep people “did 10 percent better” than when they were tested before sleeping, Stickgold says.
“Most people are crabby if they haven't had enough sleep,” and some recent findings point to sleep's role in helping us to focus on the positive and not just the negative, Stickgold says.
If sleep deprived people are given a memory task and then are allowed a good night's sleep before a test, “they only remember the negative” information, unlike those who do the task when they are not sleep deprived, he says. “So if you go through several days of lack of sleep” before making a decision, that decision will likely be skewed toward the negative, he says.
Sleep-deprived, crabby people “always leap to the worst possible conclusion,” and sleep research now suggests that's at least partly because of the kind of negative information their brains cement in memory.
When it comes to links between sleep and the mind, many mysteries remain. Brief naps, for example, should not help learning much, given what scientists believe they know about the complexities of memory formation, but recent research reveals that naps can apparently consolidate some memories as effectively as a night's slumber.
At sleep-medicine conferences, “I used to ask what people made of claims that power naps” were effective memory boosters, “and people would say, ‘That's bull. There's nothing the brain can do in 15 minutes of sleep,’” says Stickgold. “Now they say, ‘That's bull, and the naps work.”
Another fact just emerging from sleep research: People differ markedly in how vulnerable they are to having sleep loss damage their performance. “That's an uncomfortable thing to find,” says David F. Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania School of Medicine in Philadelphia. “The brain is the organ of equal opportunity,” and such findings “mean that we're not all equal” when it comes to a quality that many regard as a sign of strength — coping well when we've burned the midnight oil, he says.
— Marcia Clemmitt