People still respond to voices while sleeping

I once walked through my living room in a pair of noisy high heels, while a housemate was napping on our couch.

She yelled, in her sleep, ‘WHY IS THERE A HORSE IN THE HOUSE?’

She would be pleased to know that she is not alone in her somnolent perorations: neuroscientists now say they’ve observed people responding to verbal stimuli in almost every phase of sleep.

The study, done by researchers at Pitié-Salpêtrière University Hospital and the Sorbonne Université, both in France, is published in Nature Neuroscience.

“Our research has taught us that wakefulness and sleep are not stable states: on the contrary, we can describe them as a mosaic of conscious and seemingly unconscious moments,” says co-author Lionel Naccache.

“One of our previous studies showed that two-way communication, from the experimenter to the dreamer and vice versa, is possible during lucid REM sleep,” adds co-author Delphine Oudiette. (Lucid dreaming is when you know you’re dreaming, and REM, or rapid eye movement, sleep is the stage of sleep when most dreams occur.)

“Now, we wanted to find out whether these results could be generalized to other stages of sleep and to individuals who do not experience lucid dreams,” says Oudiette.

The researchers asked 49 people – 27 with narcolepsy and 22 without a sleep disorder – to take a nap, while monitored with polysomnography equipment which tracks brain and heart activity, along with muscle and eye movement.

People with narcolepsy feel drowsy during the day, and frequently have lucid dreams. In total, 21 of the 27 narcoleptic participants were regular lucid dreamers.

While they slept, researchers gave them a “lexical decision” test: a speaker played a human voice saying both real words and made-up words. Participants were asked to smile or frown depending on whether the word was real.

“Most of the participants, whether narcoleptic or not, responded correctly to verbal stimuli while remaining asleep,” says co-author Isabelle Arnulf.

“These events were certainly more frequent during lucid dreaming episodes, characterised by a high level of awareness. Still, we observed them occasionally in both groups during all phases of sleep.”

The researchers observed responses from participants in all bar one of the sleep stages: the only exception was participants without sleep disorders during slow-wave sleep.

Participants were also asked if they’d had a lucid dream, and if they remember interacting with someone, once they’d been woken up by an alarm.

The combination of smiling and polysomnography data allowed the researchers to predict moments during someone’s sleep when they’re more likely to respond to stimuli.

“In people who had a lucid dream during their nap, the ability to respond to words and to report this experience upon waking up was also characterised by a specific electrophysiological signature,” says Naccache.

“Our data suggests that lucid dreamers have privileged access to their inner world and that this heightened awareness extends to the outside world.”

The researchers hope to see if the patterns they’ve spotted are linked to sleep quality, and therefore whether they could be used to help treat sleep disorders.

“Advanced neuroimaging techniques, such as magnetoencephalography and intracranial recording of brain activity, will help us better understand the brain mechanisms that orchestrate sleepers’ behaviour,” says Oudiette.

They even think these windows could allow for real-time communication with sleepers, which could teach them more about sleep.