Some college students facing exams used to try to study by sleeping with
earphones on while playing a recording of material they had not managed to
cover. It was regarded as a waste of time.
Now, however, researchers at
the Weizmann Institute of Science in Rehovot believe sleeplearning of new things
may be possible – although it is not yet clear whether this would include
Prof. Noam Sobel, research student Anat Arzi and
colleagues published an article in Sunday’s latest issue of the prestigious
Nature Neuroscience on learning new information while sleeping.
found that if certain odors are presented after sounding tones during sleep,
people will start sniffing when they hear the tones alone – even when no odor is
present – both during sleep and, later, when awake. In other words, people can
learn new information while they sleep, and this can unconsciously modify their
Sleep-learning experiments are notoriously difficult to
First, researchers must be sure that the subjects are actually
asleep and stay that way during the “lessons.”
The most rigorous trials
of verbal sleep learning have failed to show any new knowledge taking root.
While more and more research has shown the importance of sleep for learning and
consolidating previous information in one’s memory, none had managed to show
actual learning of new information in an adult brain during sleep.
and Arzi – who is 31 – worked together with Sobel’s group at the Rehovot
institute’s neurobiology department with researchers from Ra’anana’s Beit
Loewenstein Rehabilitation Hospital and the Academic College of Tel
They decided to experiment with a type of conditioning that
involves exposing subjects to a tone followed by an odor, so that they soon
exhibit a similar response to the tone as they would to the odor.
pairing of tones and odors presented several advantages – neither wakes the
sleeper (in fact, certain odors can promote sound sleep), yet the brain
processes them and even reacts during slumber.
Russian physiologist Ivan
Pavlov studied conditioning of dogs to salivate when they heard a tone instead
of being exposed to food, but this experiment involved delayed
In addition, the sense of smell holds a unique non-verbal
measure that can be observed – breathing in deeply to sense an odor. The
researchers found that, in the case of smelling, the sleeping brain acts much as
it does when awake. When there is a pleasant odor, we inhale deeply, but we cut
our inhalation short when we encounter an unpleasant smell.
variation in sniffing could be recorded whether the subjects were asleep or not,
Finally, this type of conditioning, while it may appear quite simple, is
associated with some higher brain areas – including the hippocampus, which is
involved in memory formation.
In the experiments, the subjects slept in a
special lab whose walls were covered with stainless steel to prevent irrelevant
odors from being absorbed.
Their sleep state was continuously monitored.
As they slept, a tone was played, followed by an odor – either pleasant like
shampoo or unpleasant like rotten fish. Then another tone was played, followed
by an odor at the opposite end of the pleasantness scale. Over the course of the
night, the associations were partially reinforced, so that the subject was
exposed to just the tones as well. The sleeping volunteers reacted to the tones
alone as if the associated odor were still present – by either sniffing deeply
or taking shallow breaths.
The next day, the now-awake subjects again
heard the tones alone – with no accompanying odor. Although they were not aware
of having listened to them while sleeping, their breathing patterns belied
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.The team then asked whether this type of learning
is tied to a particular phase of sleep.
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. 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
learning took place during the non-REM phase.
Sobel and Arzi suggest that
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. 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.
Sobel’s lab studies the sense of smell, Arzi told The Jerusalem Post she intends
to continue investigating brain processing in altered states of consciousness
such as sleep and coma.
“The plasticity of the brain fascinates me. Once,
sleep was considered to be ‘reversible death,’ but our brains can do things when
we sleep. Now that we know that some kind of sleep learning is possible,” said
Arzi, “we want to find where the limits lie – what information can be learned
during sleep and what