Students heading back to school may be heartened to hear that you can learn in your sleep, though not perhaps in quite the way they'd hope. One study found that learned information is cemented or strengthened during sleep. Another found that we are able to form new associations to learned material while asleep.

In the first study, researchers at Northwestern University found that re-exposing yourself to that information while asleep helps, at least for people learning to play music.

Subjects made fewer errors when playing the tune they had heard while sleeping than they did while playing the other tune.

This is different from the idea that you can learn a foreign language while you sleep (hynopaedia), an idea that has gained little acceptance in the scientific community. The Northwestern researchers were looking at reinforcing previously learned information during sleep, not learning new information.

Study subjects learned to play two computer-generated tunes by pressing the appropriate series of keys on a computer keyboard. They then took a 90-minute nap and had their brain's electrical activity monitored by EEG. When the EEG showed that the subjects had entered slow-wave sleep, sleep's deepest level, one of the two tunes was played back to the sleeping subjects.

Afterwards, the subjects made fewer errors when playing the tune they had heard while sleeping than they did while playing the other tune. And the amount of improvement correlated with visible changes that could be seen in a sleeper's EEG.

Many studies have shown that sleep, especially slow-wave sleep, plays an important part in memory consolidation, the process where short-term memories are transferred into long-term storage. Even a short nap can help people retain recently learned information.

And while the Northwestern study itself looked at too short a time period to show that these memories are actually strengthened over the long run, re-exposure during sleep did make the information more accessible in the short run.

The second study found that subjects who had been exposed to pleasant or unpleasant scents paired with a tone while asleep, carried a memory of that association when awake and took deeper or shallower breaths accordingly. In other words, they had learned to connect the tone with something pleasant or unpleasant while asleep and this carried over into their waking experience.

Both studies appear in issues of Nature Neuroscience.