Sleep has long been known to have restorative properties and boost memory formation, but new research suggests that it may actually allow the brain to prune back the number of synapses — the junctions between cells that make communication between the brain and body and within the brain possible — that were formed during the day's learning experiences. Since there is a finite number of connections possible between cells, this trimming of unneeded synapses may make room for new learning to occur later on, say researchers from the Washington University School of Medicine in St. Louis.

Also surprising was the finding that a mere 16 neurons (out of the 200,000 that make up the fly brain) were involved in the learning process.

In the current study, researchers repeated methods they had used in a 2006 paper, but with one major difference: they used a strain of fruit fly in which they could track the formation of new synapses in the brain. They exposed the flies to a couple of social situations in which learning could occur. In one, male flies were released into an area with "female" flies. However, the females had either already mated, or were actually males to which female pheromones had been applied; both varieties were non-receptive to the males' advances. After a couple of days, when exposed to receptive females, the male flies did not try to mate — presumably they remembered their earlier encounters. They also slept significantly more than flies who did not undergo such an experience. In the second scenario, researchers raised flies either among other flies or in social isolation. Those raised in the socially enriched environment slept considerably more than the isolated flies.

The authors found that if the flies were allowed to sleep after learning, the number of synapses in their brains decreased, but this was not true if the flies were deprived of sleep. This indicates that sleep may be necessary for the pruning of synapses that accompanies learning.

Also surprising was the finding that a mere 16 neurons (out of the 200,000 that make up the fly brain) were involved in the learning process. Interestingly, these 16 cells are all in the lateral ventral area, which is involved in regulating circadian rhythm — that is, the sleep-wake cycle, as well as other activities that occur at designated times during the 24-hour period.

Can this research yet be applied to humans? In a broad sense, yes, say the researchers. "Right now a lot of people are worried about their jobs and the economy, and some are no doubt losing sleep over these concerns," says author Paul Shaw. "But these data suggest the best thing you can do to make sure you stay sharp and increase your chances of keeping your job is to make getting enough sleep a top priority."