If you ever tried studying for an exam by placing a recording of the material under your pillow while you sleep, you may have been disappointed in the results. Though sleeping helps your brain consolidate memories of what you learned that day while awake, it is not the optimal time for learning new material by listening to words.

However, a new Weizmann Institute study appearing in Nature Neuroscience describes how sleeping people repeatedly exposed to a tone and an odor will start sniffing when they hear that tone – even when awake, and even without the odor.

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Sense-of-smell expert Prof. Noam Sobel and research student Anat Arzi, together with Sobel’s neurobiology group and researchers from Loewenstein Hospital and the Academic College of Tel Aviv-Jaffa, experimented with a type of conditioning that does not wake the sleeping subject but does get processed by the brain in an observable way – in this case, sniffing.

The researchers found that in reaction to odors, the sleeping brain acts much as it does when awake: We inhale deeply when we smell a pleasant aroma but cut our inhalation short when assaulted by a bad smell. This variation in sniffing could be recorded whether the subjects were asleep or awake.

While this type of conditioning may appear to be straightforward, it is actually quite complex because it is associated with higher brain areas including the hippocampus, which is involved in memory formation.

Brain recalls odors smelled during sleep

The people who took part in the experiments slept in a sleep lab under continuous monitoring. Anyone who woke up, however briefly, was disqualified. As the volunteers slept, a tone was played, followed by a pleasant or unpleasant odor.

Then another tone was played, followed by the opposite kind of odor. Over the course of the night, the associations were partially reinforced by exposing the subject to the tones alone. The sleeping volunteers reacted to the tones as if the associated odor were still present – by either sniffing deeply or taking shallow breaths.

Anat Arzi and Prof. Noam Sobel taught new behaviors through the senses of hearing and smell. Photo courtesy of the Weizmann Institute
Anat Arzi and Prof. Noam Sobel taught new behaviors through the senses of hearing and smell. Photo courtesy of the Weizmann Institute

The next day, the volunteers again heard the tones with no accompanying odor. Although they had no conscious recollection of listening to them during the night, their breathing patterns showed that their brains were paying close attention. When exposed to tones that had been paired with pleasant odors, they sniffed deeply, while tones associated with bad smells provoked short, shallow sniffs.

The effects of dream amnesia on sleep-learning

The researchers were curious to find out if this type of conditioning can happen only in a particular phase of sleep.

So they did a second experiment, dividing the sleep cycles into rapid eye movement (REM) and non-REM sleep, and then induced the conditioning during only one phase or the other. Surprisingly, 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 conditioning took place during the non-REM phase.

Sobel and Arzi suspect that during REM sleep we may be more receptive to the stimuli in our surroundings, but so-called “dream amnesia” – which makes us forget most of our dreams – may just as effectively erase any conditioning that occurred in that stage of sleep. In contrast, non-REM sleep is known to be most important for memory consolidation, so it might also play a role in this form of sleep-learning.

Arzi intends to continue investigating brain processing in altered states of consciousness such as sleep and coma. “Now that we know that some kind of sleep learning is possible,” she said, “we want to find where the limits lie – what information can be learned during sleep and what information cannot.”