Humans can strengthen existing memories during sleep, but can they also learn new information while asleep? The authors exposed sleeping volunteers to pleasant and unpleasant odours, each paired with a specific tone. Subsequent exposure to the tones alone, during sleep on the same night or while awake the following morning, resulted in deep and superficial 'sniffs' for tones that were previously paired with pleasant and unpleasant odours, respectively. This suggests that humans can form new associations during sleep, without being aware of learning.