Why does our brain disconnect when we sleep?

On the train, at the beach, in front of the television... we've all experienced that moment when sleep starts to come over us: we slowly lose consciousness of our surroundings. And as we dive into the arms of Morpheus, our senses diminish; and then there's nothing. Thomas Andrillon and his colleagues studied this loss of contact with the environment. Why does the brain isolate itself during sleep?

Sleep is characterized by a loss of behavioral responsiveness. Until now, scientists weren’t able to determine exactly how neuronal activity could limit the ability to process sensory information as we sleep. In order to study this “disconnection” by the brain, a team of neuroscientists from the Department of Cognitive Studies (ENS, Paris), Pierre and Marie Curie University, Paris Descartes University, and the Technical University of Denmark sought to explore the brain’s responsiveness during sleep.

They asked 23 participants (16 women between the ages of 21 and 31) to classify words as they were falling asleep. The volunteers’ brain activity was recorded by electroencephalography. Subjects were told to press a button located to their left if the word they heard was an object and to press a button on the right if the word was an animal. The list of words (72 in all, equally divided between the 2 categories) continued as they drifted off to sleep and while they slept, even if the participants were no longer pressing the buttons. The researchers were interested in seeing to what extent the brain could still perceive and semantically analyze the information being sent.

The study revealed that during slow wave sleep (phase during which we fall asleep), the brain analyzed the meaning of the words and decided to press the correct button (but didn't actually send the motor commands). Then, during deep and REM sleep, the EEG no longer showed the characteristic peak signifying the preparation of an action in the motor cortex. This indicates that the semantic analysis of the perceived information was no longer as elaborate. Nevertheless, Andrillon’s team observed that the areas responsible for the physical analysis of the sounds (intensity, frequency...) were still active, though their activity wasn't propagated to other parts of the brain. The researchers isolated two different mechanisms depending on sleep phase.

During deep sleep, the cortex appears to respond to the words being played by further inhibiting the frontal cortex. This area is essential to complex processing, particularly understanding and following instructions. As much research has demonstrated, this sleep phase plays an essential role in consolidating memory. The brain thus needs to isolate itself from the outside world.

However, during REM sleep, brain activity mimicked normal waking activity, which they explain is likely due to the mobilization of resources for dreams, which are common during this phase. Thus, there is no longer enough energy to analyze external auditory information, which does not go beyond the auditory cortex (responsible for the rudimentary analysis of sounds). Here again, the fact that the brain goes into “vigilance” and "responsive" rest modes is beneficial.

According to the authors: “Sleep can thus be seen as a self-regulated process in which external information can be processed in lighter stages but suppressed in deeper stages.” And it’s for the good of our brains.
Source: T. Andrillon et al., Neural Markers of Responsiveness to the Environment in Human Sleep, in The Journal of Neuroscience, 15 June 2016.

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