According to researchers of the study, people who take a nap and dream about a task they've just learned perform it better upon waking than either those who don't sleep at all or those who sleep but don't report any associated dreams.
The learners in the study were asked to sit in front of a computer screen and learn the layout of a three-dimensional maze so that they could find their way to a landmark (a tree) when they were plopped down at a random location within the virtual space five hours later.
Those who were allowed to take a nap and also remembered dreaming of the task found the tree in less time.
"We at first thought that dreaming must reflect the memory process that's improving performance. But when you look at the content of the dreams, it was hard to argue that," said Robert Stickgold of Harvard Medical School.
In a couple of cases, the dreamers said they recalled just the music from the computer maze.
One subject said they were dreaming that there were people at particular checkpoints in the maze, even though the real maze didn't have any people or checkpoints.
Another said they dreamt about an experience they'd had tromping through bat caves and thinking that the caves were like mazes.
"We think that the dreams are a marker that the brain is working on the same problem at many levels. The dreams might reflect the brain's attempt to find associations for the memories that could make them more useful in the future," Stickgold said.
In other words, it's not that the dreams led to better memory, but rather that they are a sign that other, unconscious parts of the brain were working hard to remember how to get through the virtual maze. The dreams are essentially a side effect of that memory process.
The new study was reported online on April 22nd in Current Biology, a Cell Press publication.