Researchers from the Weizmann Institute in Israel found that if certain odours are presented after tones during sleep, people will start sniffing when they hear the tones alone - even when no odour is present - both during sleep and, later, when awake.
The study chose to experiment with a type of conditioning that involves exposing subjects to a tone followed by an odour, so that they soon exhibit a similar response to the tone as they would to the odour.
The pairing of tones and odours presented several advantages. Neither wakes the sleeper (in fact, certain odours can promote sound sleep), yet the brain processes them and even reacts during slumber.
Moreover, the sense of smell holds a unique non-verbal measure that can be observed - namely sniffing. The researchers found that, in the case of smelling, the sleeping brain acts much as it does when awake: We inhale deeply when we smell a pleasant aroma but stop our inhalation short when assaulted by a bad smell.
This variation in sniffing could be recorded whether the subjects were asleep or awake. Finally, this type of conditioning, while it may appear to be quite simple, is associated with some higher brain areas - including the hippocampus, which is involved in memory formation.
In a second experiment, they divided the sleep cycles into rapid eye movement (REM) and non-REM sleep, and then induced the conditioning during only one phase or the other.
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They found that the learned response was more pronounced during the REM phase, but the transfer of the association from sleep to waking was evident only when learning took place during the non-REM phase.
During REM sleep we may be more open to influence from the stimuli in our surroundings, but so-called "dream amnesia" - which makes us forget most of our dreams - may operate on any conditioning occurring in that stage of sleep, researchers said in a statement.
In contrast, non-REM sleep is the phase that is important for memory consolidation, so it might also play a role in this form of sleep-learning.
The study was published in journal Nature Neuroscience.