“To do science you have to have an idea, and for years no one had one; they saw sleep as nothing but an annihilation of consciousness. Now we know different, and we’ve got some very good ideas about what’s going on.”
Throughout the ages, scientists and thinkers have been trying to determine why people have to sleep. But so far all they know is what any parent of a newborn quickly discovers: sleep loss makes it harder to cope with stress, our thoughts more mentally foggy and our bodies more prone to get sick.
Neuroscientists have long wondered if sleep is somehow tied to learning and memory, or other cognitive processes. Now new findings suggest that sleep does indeed appear to play a crucial role in sorting and storing our memories. Harvard and McGill Universities have reported that participants who napped after playing a memory game score significantly higher on a retest than those who did not sleep.
“We think what’s happening during sleep is that you open the aperture of memory and are able to see this bigger picture,” said the study’s senior author, Matthew Walker, a neuroscientist who is now at the University of California, Berkeley. He added that many such insights occurred “only when you enter this wonder-world of sleep.”
But the theory that a sleeping brain can do things an awake brain can’t is still controversial. But the new research highlights a transformation in the way scientists are now viewing the sleeping brain. It was once seen as a blank screen, but has now emerged as an active, secretive intelligence that comes out for its “nightshift” to do some serious work.
Researchers say that studies are even showing that taking naps improves memory as well.
“We are finding that if a person takes a nap that contains slow-wave sleep — deep sleep — that performance on declarative memory tasks, which require the memorization of fact-based information like word-pairs, is enhanced compared to a person who doesn’t take a nap,” researcher Matthew Tucker said.
Previous studies of nocturnal sleep have found the same thing. In one 2003 study, Sara Mednick, then at Harvard and now at the University of California, San Diego, led a team that had 73 people come into the lab at 9 a.m. and learn to discriminate between a variety of textured patterns. Some of the participants then took a nap of about an hour at 2 p.m. and the others did not.
When retested at 7 p.m. the rested group did slightly better. When tested again the next morning, after everyone had slept the night, the napping group scored much higher. The naps included both REM and deep sleep.
“We think that a nap that contains both these states does about the same for memory consolidation as a night’s sleep,” when it comes to pattern recognition learning, Dr. Mednick said.
In series of experiments that he began in the early 1990s, Dr. Carlyle Smith of Trent University in Canada has found a strong association between the amount of Stage 2 sleep a person gets and the improvement in learning motor tasks. Mastering a guitar, a hockey stick or a keyboard are all motor tasks.
Musicians, among others, have sensed this instinctually. A piece that is difficult during and evening practice will often flow better in the morning for some reason. But only in recent years has the science caught up and given their hunch some scientific backing.
For instance, Dr. Smith said that people typically got most of their Stage 2 sleep in the second half of the night. “The implication of this is that if you are preparing for a performance, a music recital, say, or skating performance, it’s better to stay up late than get up really early,” he said in an interview. “These coaches that have athletes or other performers up at 5 o’clock in the morning, I think that’s just crazy.”
So here’s the big question: is something going on with memory processing that is unique to sleep?Skip to next paragraph
Subimal Datta, a neuroscientist across the river at Boston University School of Medicine, says yes. In his studies of animals, he has documented that during sleep the brain is awash in a chemical bath unlike any during waking. Levels of inhibitory transmitters increase sharply, and levels of many activating messengers drop, or shut down entirely.
Even before REM is detectable, Dr. Datta said, a small pocket of cells in the brainstem spurs a surge in glutamate — an activating chemical — which leads to protein synthesis and other changes that support long-term memory storage.
“During waking we have a thousand things happening at once, the library is filling up, and we can’t possibly process it all,” Dr. Datta said. While awake the brain is also gathering lots of valuable information subconsciously, he said, without the person’s ever being aware of it.
“It’s during sleep that we have this special condition to clear away this overload, and these REM processes then help store what’s important,” Dr. Datta said.
Dreams still defy scientific understanding but they also appear to play a role in the evolving theory of sleep-dependent learning.
Some scientists argue that during REM sleep, or dream mode, the brain will mix, match and make sense of the memory traces it has preserved, as it looks for connections that help make sense of life.
It was during sleep that the Russian scientist Dmitri Mendeleev was reported to have developed the periodic table of the elements. Friedrich August Kekule, a 19th-century chemist, said he worked out the chemical structure of the benzine ring after dreaming of a snake biting its tail. Athletes like Jack Nicklaus, have also mentioned insights discovered while sleeping.
“It does make sense these insights come during REM,” Dr. Walker said. “I mean, what better time to play out all these different scenarios and solutions and ideas than in dreams, where there are no consequences?”
The problem, he says, is how to study it. Few of us would venture to say we know what goes on in our heads while sleeping, but to quantify and generalize such an elusive state has been quite a journey.
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