“Imagine that you wake up in the morning feeling nothing special, yet you find yourself inexplicably behaving just a bit differently during the day. For example, you take a sniff every time you hear a tone,” says Prof. Noam Sobel. Of course, the people this actually happened to knew they had volunteered for a sleep experiment in Sobel’s lab. They knew that their sleep patterns had been closely monitored. But they had no recall, whatsoever, of the “lesson” they had learned while snoring peacefully. It was the sniffing that gave it away: While asleep, they had undergone conditioning to associate tones with smells. The next day, when the now awake volunteers heard the tones, their unconscious sniffs revealed whether they have been conditioned to associate the tone with a nice smell or a nasty one.

This may be the first incontrovertible demonstration of sleep learning in human adult brains. The trick, says Anat Arzi, a PhD student in Sobel’s group who led the project, was to find the right teaching method.

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Tests of verbal learning (think of the old tape recorder-under-the-pillow experiments) had pretty much ruled this out as way to absorb lessons painlessly in one’s sleep. And the results of other experiments either suggested that the subjects had briefly woken during the trials, or else they took place during unnatural sleep, with the subjects drugged to make sure they stayed out for the duration. So, on the one hand, years of sleep learning research had left the question up in the air and tending toward falling on the side of nonexistence. But on the other hand, a number of recent studies have suggested a close connection between sleep and learning and memory consolidation. “In spite of all the previous research, we thought that some kind of sleep learning should be possible,” says Arzi. “The question was: which kind?”

Tones and smells turned out to be ideal: They don’t wake the sleepers, yet they are sensed during sleep. And sniffing – the reflexive response to the odors – occurs whether one smells them asleep or awake. That meant that instead of relying on reported memories, the researchers only had to watch for the long, deep sniff we automatically take when we smell a good smell, or the short, shallow one associated with a bad smell to know that the subjects had been conditioned to associate a tone with a particular odor.

What’s next? Sobel and Arzi found that the conditioning is best retained when it takes place during non-REM sleep – the same sleep stage in which the things learned during the day get consolidated into memory. So there is an intriguing connection there that they want to explore. And, while we’ll never be able to study for that physics exam in our sleep, finding proof for one type of sleep learning suggests that others might be possible.