It is well known that sleep has an important role in strengthening existing memories, but according to research published today in Nature Neuroscience, we can also learn entirely new information while we snooze1.

Anat Arzi of the Weizmann Institute of Science in Rehovot, Israel, and her colleagues used a simple form of learning called classical conditioning to teach 58 healthy participants to associate odours with sounds as they slept.

They repeatedly exposed the sleeping participants to pleasant odours, such as deodorant and shampoo, and unpleasant odours such as rotting fish and meat, and played a specific sound to accompany each scent.

This conditioning was already known to alter sniffing behaviour when taught to people who are awake. Subjects who learn such associations sniff strongly when they hear a tone associated with a pleasant smell, but only weakly in response to a tone associated with an unpleasant one.

The researchers found that their participants could also learn these conditioned responses in their sleep, and that the learning persisted even after they woke up, such that they would sniff strongly or weakly on hearing the relevant tone — even if there was no odour. This occurred regardless of the stage of sleep during which the learning procedure was carried out. However, the sniffing responses were slightly more pronounced in those participants who learned the association during the rapid eye movement (REM) stage, which typically occurs during the second half of a night's sleep. 

Pillow power

Arzi thinks that we could probably learn more complex information while we sleep. “This does not imply that you can place your homework under the pillow and know it in the morning,” she says. “There will be clear limits on what we can learn in sleep, but I speculate that they will be beyond what we have demonstrated.”

The findings show that the brain can process the pleasantness of odours while we are asleep, and that the information is retained on waking — despite the fact that the participants were completely unaware that they had learned the relationship between smells and sounds.

“This study is important because it reliably shows that a type of associative learning requiring awareness of the relationship between the stimuli can, in fact, be established during unconscious states,” says Tristan Bekinschtein, a neuroscientist at the UK Medical Research Council's Cognition and Brain Sciences Unit in Cambridge. “It remains to be seen if the neural networks involved in this learning are similar to the ones recruited during wakefulness,” he adds.

In 2009, Bekinschtein and his colleagues reported that some patients who are minimally conscious or in a vegetative state can be classically conditioned to blink in response to air puffed into their eyes. Conditioned responses such as these could eventually help clinicians to diagnose these neurological conditions, and to predict which patients might subsequently recover.

The findings by Arzi and her colleagues might also be useful for these purposes, and could lead to 'sleep therapies' that help to alter behaviour in conditions such as phobia.

“We are now trying to implement helpful behavioural modification through sleep-learning,” says Arzi. “We also want to investigate the brain mechanisms involved, and the type of learning we use in other states of altered consciousness, such as vegetative state and coma.”