A new study by an international research team, involving Monash University, has found that sleepers actively suppress important information each time their eyes move during rapid eye movement (REM) sleep.

Thomas Andrillon, PhD, research fellow at the Turner Institute for Brain and Mental Health at Monash University, says the study findings are interesting given that eye movements in REM sleep usually mark the occurrence of dreams.

Dreaming could represent a form of maximum inattention, he says, where sleepers get so absorbed by their own fantasy that they become incapable of processing their external world, consciously or unconsciously.

The findings, published in the international journal Current Biology, have implications for REM sleepers who remain unaware of “threat signals” while dreaming as the mind confines salient information as background noise and, therefore, maintains the body’s restful state.

“An interpretation of these results is that, when we dream, we start processing internally-generated percepts (the stuff of dreams) which will compete with external information. The result of this competition would be the incapacity to process external sensory inputs, such as a baby crying in another room or someone breaking into the family home,” Andrillon says in a release.

“Dreaming could be a deceptive state in which we feel we could wake up easily but we are completely missing out on what’s happening around us.”

Sleep is divided into two distinct phases. In non-rapid eye movement (NREM) sleep, the brain slows down and becomes hyper-synchronized where dreams are typically more infrequent and less complex.

Rapid eye movement sleep is the second phase. In this state, sleepers seem fast asleep but their brain activity resembles wake activity and their eyeballs can move frenetically behind their closed eyelids, as if sleepers were exploring an inner world.

“Sleep is actually a busy and vital time for the brain and past research has shown the importance of sleep in the consolidation of memories, the cleaning up of metabolic waste and even on the strengthening of the immune system,” he says.

“The brain can still track our environment, and select and process the most relevant sources of information. This could be useful if you take a brief nap during your commute to work. But, think twice before sleeping through meeting—your sleeping brain is not as smart as you are!”

The research team, led by eminent cognitive neuroscientist Professor Sid Kouider from the École Normale Supérieure in France, monitored the sleep patterns of 18 participants while they listened to competing pieces of audio.

As the participants fell asleep, researchers played stories in both ears. They delivered two stories at the same time, one in each ear, as if sleepers were in a room with two people talking in separate conversations. Except, one conversation was logical, the other was devoid of meaning.

Participants’ brain activity was recorded through an electroencephalogram—a process whereby electrodes are placed on the scalp, which allows researchers to almost reconstruct the happenings in a person’s mind.

Using this approach, researchers found that the brain not only processes both competing messages, but intelligible speech was prioritised over meaningless dialogue. This is much the same way our brain works during wakefulness.

However, this ability to prioritise relevant, meaningful information stopped and reversed whenever eye-movements were recorded. This reversal suggests a critical change in the way we process our environment when we dream in favour of a more complete isolation from the outside world.

“Through this research, we have not only tried to examine how the dreaming brain manages the interplay between real and imagined sensory information, but to also understand why we dream in the first place,” Andrillon says.

“Our results further indicate how the dreams themselves could isolate sleepers from their environment. Such isolation could come at a price, including missing cries of a newborn while dreaming, despite being fully conscious.

“But, isolating ourselves from our surroundings could have important advantages in terms of memory consolidation, emotional balance, and making sure that we wake up every morning fresh and ready to face up to a challenging world—and a very real one.”