"In general, there's considerable evidence suggesting that sleep promotes the fixation of long-term memories of things that were learned before going to sleep," said McGaugh.
Sleep-learning was widely discredited in the 1950s, so the fact that CDs are available says something about the appeal of the idea.
"The information is just not processed that way while you're asleep," said Scott Lilienfeld, a professor of psychology at Emory University who discussed the myth in his new book, "50 Great Myths of Popular Psychology: Shattering Widespread Misconceptions about Human Behavior."
The problem with the original research that suggested the possibility of sleep learning was that researchers didn't do enough to make sure the subjects were actually asleep.
"When studies later controlled for that fact by making sure subjects were in fact asleep…whatever affects there were disappeared," said Lilienfeld. "It shows why good scientific control was important."
While he said studies like the Northwestern one are important, he does worry that it may be misused by a misunderstanding of the science.
"The danger of this kind of thing -- is that then advocates of sleep assisted learning can misuse them, lead the public to think they can learn new things while they're sound asleep," said Lilienfeld. "It does not say anything about learning new information -- it does not mean or imply that one can learn entirely new information during sleep, nor do the results bear on learning complex information, like entirely new words, let alone languages or concepts."
For someone thinking of buying any sleep-learning device, Rudoy had this advice: "If someone comes to you tomorrow and tries to sell you something, I would be skeptical," he said.