It's not just the amount of sleep we get that is so important for learning, but the quality of that sleep. That's according to a new study that made precise use of beeping noises to disrupt deep "slow-wave" sleep among 13 elderly participants (average age 60 years), without actually waking them up.
The beeping was used in such a way that although the participants' were deprived of deep sleep, their total sleep time and number of sleep stages were unaffected (compared with a comparison night of undisturbed sleep).
After a night of either shallow or deep sleep, the participants had their brains scanned while they viewed 50 images of houses and landscapes. The next day they had to say which of 100 images were repeated from the day before. The participants' performance was superior when a night with deep sleep had preceded the learning of the images, compared with a night of shallow sleep, even though total sleep time was the same in each case (36.6 images correctly identified versus 31.4 images, on average).
Moreover, the brain scans showed that during the initial viewing of images, activity in the hippocampus, the seat of human memory, was reduced after shallow versus deep sleep, but only for those images that were subsequently recalled. This suggests that shallow sleep somehow interferes with the way the hippocampus encodes new, explicit memories.
By contrast, so-called "implicit memory", appears to be unaffected by sleep quality. Regardless of the kind of sleep they'd had, participants showed superior performance at a sequence learning task when the sequence was fixed rather than random, even though they were consciously unaware of what the actual sequence was.
"The mechanism by which deep sleep affects hippocampal function is unclear," Ysbrand Van Der Werf and colleagues said, "but may involve local synaptic changes resulting from slow wave activity."
Ysbrand D Van Der Werf, Ellemarije Altena, Menno M Schoonheim, Ernesto J Sanz-Arigita, Jose´ C Vis, Wim De Rijke, Eus J W Van Someren (2009). Sleep benefits subsequent hippocampal functioning. Nature Neuroscience. In Press.