In fact, among all the animals, humans don't have especially high or low amounts of REM sleep, he tells WebMD. "We really don't know the purpose of REM sleep," Siegel says. "We don't even fully understand learning."
REM sleep may actually help us with certain types of learning -- what's called "procedural" learning, "when you're learning how rather than what," says Robert Stickgold, PhD, assistant professor of psychiatry at Harvard Medical School and a cognitive neuroscientist.
Stickgold agrees with Siegel, that "simple declarative memory" -- like remembering facts for a test, phone numbers, or word lists -- is not dependent on sleep. "Everything we learn while waking gets learned before we sleep. So clearly those are not dependent on sleep for their initial consolidation," he tells WebMD.
Studies of procedural learning, however, "show clear evidence that you don't get improvement until you sleep," Siegel says.
A perfect example: studies show that when learning muscle-related tasks -- like dancing, piano playing, gymnastics -- there is significant improvement after a night's sleep, he tells WebMD.
This type of learning occurs in the cortex of the brain, Siegel explains. "The hippocampus hands out the 'episodic details' like phone numbers, what you had for breakfast this morning." The facts you crammed for today's test are likely delivered via the hippocampus.
But it's the neocortex that's working when you need to re-process information, as in learning muscle-related tasks. It's also the neocortex that will provide the answer when someone asks what you would like for breakfast, he says.
"Your neocortex would process all the information -- the fact that you prefer waffles, but the last couple of times you had them they were kinda soggy," Stickgold tells WebMD.
It's also your neocortex that makes "sleeping on a problem" -- a concept that seems known across all cultures -- so effective, he says. "You're offered the perfect job, but it's not in an ideal city. You can't decide which college is right for you. Everybody knows that you can count on the fact that if you go to bed confused, when you wake up, you'll have the answer, or the answer isn't to be found."