Sleep learning experiments are especially difficult conduct, researchers have to make sure the participants are actually asleep. The team at the Weizmann Institute of Science exposed sleeping subjects to a tone followed by a smell, neither of which woke them up. The odor was either pleasant of unpleasant – generating a deep or a shallow breath – and each was associated with a particular tone. When the corresponding tones were replayed, the participants either elicited a deep or shallow breath, without the presence of the odor. When exposed to tones that had been paired with pleasant odors, they sniffed deeply, while the second tones – those associated with bad smells – provoked short, shallow sniffs.