A new theory about sleep's benefits for the brain gets a boost from fruit flies in Science. Researchers at Washington University School of Medicine in St. Louis found evidence that sleep, already recognized as a promoter of long-term memories, also helps clear room in the brain for new learning.

The critical question: How many synapses, or junctures where nerve cells communicate with each other, are modified by sleep? Neurologists believe creation of new synapses is one key way the brain encodes memories and learning, but this cannot continue unabated and may be where sleep comes in.

"There are a number of reasons why the brain can't indefinitely add synapses, including the finite spatial constraints of the skull," says senior author Paul Shaw, Ph.D., assistant professor of neurobiology at Washington University School of Medicine in St. Louis. "We were able to track the creation of new synapses in fruit flies during learning experiences, and to show that sleep pushed that number back down."

Scientists don't yet know how the synapses are eliminated. According to theory, only the less important connections are trimmed back, while connections encoding important memories are maintained.

Many aspects of fly sleep are similar to human sleep; for example, flies and humans deprived of sleep one day will try to make up for the loss by sleeping more the next day. Because the human brain is much more complex, Shaw uses the flies as models for answering questions about sleep and memory.

Sleep is a recognized promoter of learning, but three years ago Shaw turned that association around and revealed that learning increases the need for sleep in the fruit fly. In a 2006 paper in Science, he and his colleagues found that two separate scenarios, each of which gave the fruit fly's brain a workout, increased the need for sleep…