Sleep is already known to boost our brains' ability to learn and remember -- and now, a new study actually demonstrates this ability.

Researchers at the Weizmann Institute of Science played a number of tones to study participants as they slept during the night. After playing each tone, the researchers presented an odor to the still-sleeping participants. Some of the tones only had pleasant odors released after them, while other tones only had unpleasant odors released after them. The still-sleeping study participants reacted by either taking in a deeper breath when there was a pleasant smell, or taking a shallow breath if there was an unpleasant smell.

Then, as the night went on, the researchers tried just playing the tones for the sleeping study participants, without presenting the odors afterward. Even though there was no odor, the sleeping participants still sniffed -- either shallow breaths if it was after a tone that had previously been linked with the unpleasant smell, or a deeper breath if it was a tone that had been previously linked with a pleasant smell.

This held true even after the study participants woke up, too -- when they were awake, the researchers played the tones for them. They had the same breathing reactions as when they were sleeping.

"This acquired behavior persisted throughout the night and into ensuing wake, without later awareness of the learning process. Thus, humans learned new information during sleep," the researchers wrote in the Nature Neuroscience study.

The researchers said that this sniffing -- even when there was nothing to sniff -- showed that their brains were processing the link between tone and smell, even as they were sleeping.

Last year, a study from University of California, Berkeley, researchers found that the dreamless sleep stage that can take up half the night -- known as stage two of non-REM sleep -- may actually be the time when our brains are readying themselves and clearing space for learning by moving memories from the hippocampus to the prefrontal cortex.

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