Can't remember where you put your keys? Here's a tip for you: record the sound of your jingling keys and put it into your iPod. Then get someone to play it to you at night, while you are deep asleep. Chances are it'll help you remember.
That's the conclusion of a recent report in the journal Science by neuroscientists at Northwestern University, who carried out a small study, with 12 volunteers, to figure out whether specific sounds played during sleep would boost the memory of information learned while awake.
First, the participants were asked to memorize the correct location of 50 images on a computer monitor. The images were shown one at a time, arranged in a random place on the screen a cat appeared on the bottom left, a gong on the top right and so forth. Each object was shown with a related sound so subjects heard a meow with the picture of a cat, and a crashing noise coupled with the image of a gong. After studying the 50 images and locations, the participants were asked to take a short nap in a recliner in an adjacent room.
Volunteers for the study weren't hard to find, notes Ken Paller, professor of psychology at Northwestern University and lead author of the paper: "People are happy when they are paid to have a nap."
The volunteers were outfitted with electrode caps akin to a white shower cap with a jungle of wires sticking out of it that tracked their brain waves in order to determine their stage of slumber. Using an electroencephalogram (EEG), investigators monitored the sleepers' brain activity, and just when the squiggly lines on the screen showed that participants had entered deep sleep, researchers began playing a series of 25 of the sounds that the individual had heard earlier in the memory game. "[The volume] was a little over a whisper, probably much [quieter] than ... your iPod," says John Rudoy, one of the study's authors and a graduate student at Northwestern.
When the participants woke up about an hour later, they said they hadn't heard a thing. But the test results suggested otherwise. On average, each person did slightly better at remembering the correct locations of the 25 objects whose related sounds had been cued during sleep than those of the other objects. The sounds appeared to have entered the sleeping brain and helped consolidate associated memories.
Many researchers who study sleep and memory were excited by this new study (not to mention purveyors of nighttime subliminal-message CDs that purport to make you quit smoking or love yourself) but experts acknowledge that more work needs to be done. "I would consider this a very, very small effect," says Paller, so don't expect to be able to boost your SAT score while sleeping just yet.
For nearly two centuries, researchers have suspected that sleep plays an important role in learning and memory. But it's only in the last decade that neuroscientists have discovered the most convincing evidence that memory is indeed dependent on sleep. The prevailing theory is that during deep sleep, the brain replays certain experiences from the day, which, in turn, strengthens the memory of what happened. It is thought that when it comes to factual memories, like names, faces, numbers or locations, memory consolidation happens only during deep sleep a phase of nonrapid eye movement sleep. (The other broad type of sleep, called rapid eye movement or REM sleep, which is when dreaming occurs, is believed to play a role in consolidating memories involving emotions and motor skills, such as dancing or playing an instrument.)
A minority of neuroscientists disagree that sleep actively aids memory, suggesting instead that consolidation of memories is merely a side effect of snoozing. They argue that the true purpose of sleep is to "cool down" the brain by resting the neurons that have been firing all day long. Paller's study lends support for the majority view: when sounds were played to the sleeping brain, the EEG patterns indicated activity signaling that perhaps certain memories were being revisited and this processing appeared to strengthen memories. "The meow somehow stimulated the association of the cat with a certain position on the screen," suggests Jan Born, a memory and sleep researcher at the University of Lübeck in Germany, who was not involved in the new study.
Born and his team have also been able to influence memory recall during sleep not with sounds, but with odors. In that study, published in March 2007 in Science, researchers asked people to play a memory card game while the smell of roses wafted through a special face mask. Later that night, when the participants were fast asleep, the same odor was delivered to some of them. The following morning, each person played the same game, and the results were clear: the players who got the nighttime rose odor were significantly better at remembering the card pairs than the group who smelled nothing.
Commenting on the new paper, Born suggests that using sounds is more effective than smells because it lets you choose the memories you want to promote. "Auditory stimuli have the advantage that they can be very specifically linked to visual stimuli," says Born. "With odors, this kind of thing is not possible."
Beyond sensory stimulation during sleep, the timing of sleep may also be important to memory. Recent research suggests that deep sleep can strengthen factual memories, but only if the person naps within 12 hours of learning. In other words, if you have to memorize an SAT word list, you might be better off doing it at night rather than in the morning.
Although researchers are still a long way from understanding exactly how sleep affects memory, they are certain that getting too little sleep is a detriment. A 2007 paper in Nature Neuroscience reported that in addition to consolidating recently learned memories, "sleep before learning is equally important in preparing the brain for next-day memory formation." The study found that people who had skipped a night's sleep fared worse at making new memories the next day, compared with those who had gotten a good night's sleep. Turns out, Mom's advice may have been right all along.