The students who took part in the evening training sessions performed better the next morning after sleeping, than they did after being trained.
The study findings were published in the current issue of the journal Learning & Memory.
Nusbaum believes that people encounter "lots of interference" during the course of a day, and "you forget some of what you've learned." But researchers don't yet know what causes the interference.
The role that dreams play in the learning process -- if any -- isn't clear either. But some dreams could serve as a kind of practice for the brain, Nusbaum said.
"If you play a video game a lot, and you're playing in your dreams, maybe that could help you learn," he said.
Jerry Siegel, professor at the Center for Sleep Research at the University of Calfornia, Los Angeles, said going without sleep hurts performance, but he's not convinced that sleep itself actively contributes to learning.
"If you take a break for a few hours, it can easily be shown that learning did occur, because performance is better at the start of a new learning session than it was at the end of the initial session," he said. "No sleep needs to occur for this to happen."
Still, Siegel, who reviewed the new study, suggested that sleep before learning a skill is crucial.
"For long-term retention, it is more important to be well rested and therefore attentive when you are doing the learning than afterwards," he said. "It is even better if you don't have to choose and get your natural amounts of sleep every day."
Get basic information about sleep from the University of Washington.
SOURCES: Howard Nusbaum, Ph.D., professor of psychology, University of Chicago; Jerry Siegel, Ph.D., professor of psychiatry and
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