Naps, Learning and REM

It's a shame that we stop encouraging naps once the preschool years are over. After all, there's a growing body of scientific evidence that the afternoon siesta is an important mental tool, which enhances productivity, learning and memory. (It's really much more effective than a cup of coffee.) Here's the Times:

Have to solve a problem? Try taking a nap. But it has to be the right kind of nap -- one that includes rapid eye movement, or REM, sleep, the kind that includes dreams.

Researchers led by Sara C. Mednick, an assistant professor of psychiatry at the University of California, San Diego, gave 77 volunteers word-association tests under three before-and-after conditions: spending a day without a nap, napping without REM sleep and napping with REM sleep. Just spending the day away from the problem improved performance; people who stayed awake did a little better on the 5 p.m. session than they had done on the 9 a.m. test. Taking a nap without REM sleep also led to slightly better results. But a nap that included REM sleep resulted in nearly a 40 percent improvement over the pre-nap performance.

The study, published June 8 in The Proceedings of the National Academy of Sciences, found that those who had REM sleep took longer naps than those who napped without REM, but there was no correlation between total sleep time and improved performance. Only REM sleep helped.

Numerous studies have now demonstrated that REM sleep is an essential part of the learning process. Before you can know something, you have to dream about it.

This outlandish notion began with a scared rabbit. In the early 1950s, scientists at UCLA discovered that the rabbit hippocampus, when aroused by some fearful stimulus in its environment (a coyote, for example), would start pulsing with a very distinctive beat, which they dubbed the "theta rhythm." Subsequent studies found the same beat in several species, but only when the animals were extremely excited or scared or were engaged in an active motor movement. Rats exuded a theta rhythm whenever they were exploring their cage. Cats had it stalking prey. But what was this rhythm's function? Why did the hippocampus--a brain structure involved in learning and memory--become active in this way during moments of intense awareness? Stumped, the UCLA scientists shelved their work and moved on to other things.

Years later, Case H. Vanderwolf of the University of Western Ontario made an even stranger discovery: Theta rhythm was also present during sleep--activity and rest provoked the same strange brain activity. Nobody could figure out the meaning of this.

The breakthrough came in 1972, when psychologist Jonathan Winson came up with a simple theory: The rabbit brain exhibited the same pattern of activity when it was scared and when it was dreaming because it was dreaming about being scared. The theta rhythm of sleep was just the sound of the mind processing information, sorting through the day's experiences and looking for any new knowledge that might be important for future survival. They were learning while dreaming, solving problems in their sleep.

Winson's theory was ridiculed. At the time, most scientists assumed that our dreams were accidents of the brain stem, nothing more than a Dadaist montage of meaningless hallucinations. But Winson maintained that this hypothesis made no sense. For one thing, our dreams don't seem random. Instead, they unfold in intricate narrative scenarios, which tend to reflect our daily activities. According to Winson, these nighttime stories--that flurry of theta rhythm--were actually carefully scripted events, in which our new knowledge was put to the test. Did our new learning help us solve our invented problems? Was it a good "survival strategy?" If the answer was yes, then the knowledge was woven into the brain. We woke up a smarter person. The rabbit figured out how to escape its predator. We also, therefore, learn by pretending to do.

Whatever the elegance of Winson's theory, he lacked conclusive evidence. What he needed was a study that directly linked a brain's real-world experience with its manufactured dreams. That study arrived in 2001, when Matthew Wilson, a professor at MIT, published a paper in Neuron ("Temporally Structured Replay of Awake Hippocampal Ensemble Activity during Rapid Eye Movement Sleep") about the dreams of rats.

Wilson began his experiment by training rats to run through mazes. While a rat was running through one of these labyrinths, Wilson measured clusters of neurons in the hippocampus with multiple electrodes surgically implanted in its brain. As he'd hypothesized, Wilson found that each maze produced its own pattern of neural firing. To figure out how dreams relate to experience, Wilson recorded input from these same electrodes while the rats were sleeping. The results were astonishing. Of the 45 rat dreams recorded by Wilson, 20 contained an exact replica of the maze they had run earlier that day. The REM sleep was recapitulating experience, allowing the animals to consolidate memory and learn new things. Wilson's lab has since extended these results, demonstrating that "temporally structured replay" occurs in both the hippocampus and visual cortex.