Forget Freud's theory that dreams are the expression of subconscious desires. Instead, the brain seems to need dream sleep for a more G-rated reason: to remember. A new study of brain activity suggests that people replay the day's events and solidify memories while sleeping.
The link between learning and sleep has been studied for decades. In many animals, including humans, sleep deprivation has a devastating effect on memory, particularly for recently acquired skills. Disrupting REM sleep--the sleep stage when dreams occur, named for the rapid eye movements that characterize it--has the worst effect on memory in people. Another line of evidence comes from studies of sleeping rats. The rats replay the neuronal firing patterns that were triggered when they explored new environments shortly before drifting off. But no one had looked to see if the same was true in people.
To find out, a team of researchers led by Pierre Maquet of the University of Liege in Belgium used positron emission tomography (PET) to get a three-dimensional picture of brain activity. The researchers first scanned a group of seven subjects while they learned a task that required them to push a sequence of computer keys. A constellation of brain areas lit up. During REM sleep, some of these same brain areas were more active in people who had learned the new skill than in people who hadn't learned it, the team reports in the August issue of Nature Neuroscience. Co-author Axel Cleeremans of the Universite Libre de Bruxelles speculates that this replay puts memories into permanent storage.
The findings are "stunning," says cognitive neuroscientist Robert Stickgold of Harvard University. "I'm surprised that the reactivation is so visible in brain imaging." The heightened activity means that the body is spending an immense amount of energy to reinforce learning during sleep, he says.