The sleeping brain perceives external signals differently at different stages of sleep. Sleep blocks all external signals - they cannot go beyond a special area of the brain called the thalamus, through which information flows from almost all senses (except the olfactory one).
At the same time, since the 60s of the last century, it has been known that some signals still pass through the "sleep block", for example, a person can hear his name. What exactly happens in the brain is not yet clear, but neuroscientists managed to find out a number of interesting details.
So, two years ago, researchers from the Parisian Higher Normal School found that during daytime sleep a person hears words that someone says next to him, and if he is asked to press a button on some words - for example, on the names of animals - he will press it without waking up. Moreover, according to the electroencephalogram, it was possible to notice that the motor cortex, which is responsible for movement, prepares for the fact that the agreed word "will enter" the brain.
In a new article published in The Journal of Neuroscience, Thomas Andrillon and his colleagues describe another experiment that was set up at night. As you know, sleep is divided into two phases alternately replacing each other, slow (which itself is still divided into several stages) and fast (or REM sleep). While the volunteers slept, their brain activity was assessed during REM sleep, shallow NREM sleep, and deep NREM sleep.
The picture turned out to be quite complex. If during the slow shallow stage the motor cortex was preparing to move a finger in response to the right word, then with the onset of the deep stage all such activity in the brain ceased. When it was the turn of REM sleep, the motor cortex was ready to move again, but only if the required word did not appear for the first time, that is, if the sleeping brain had already had the opportunity to think about it.
Obviously, during the shallow stage of slow wave sleep, information not only somehow seeps through the sleep barrier, but also undergoes some kind of analysis. With the "deepening" of sleep, the activity of neurons is already strongly synchronized, they all fall asleep and cannot analyze anything. With REM sleep, the processing of external information is possible, but then its own violent activity of the brain arises, which does not give the sleeper from the first attempt to hear and understand what he heard.
The fact that the sleeping brain perceives external information during the shallow stage of NREM sleep has been suspected by neuroscientists for a long time, but the features of the REM phase and deep NREM sleep have never been observed in this form.
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Speaking about the fact that the sleeping brain perceives what is happening “outside”, one cannot but recall a very recent article about “one-hemispheric sleep” in people: as it turned out, in a new place we sleep badly because one of our hemisphere remains awake and hears what's going on around.
Presumably, this is an old evolutionary legacy that we inherited from the times when a person who fell asleep in a new place was faced with various unknown dangers.