Just like cats, dogs can spend many hours sleeping. The time they spend asleep enables them to recuperate from exertion, as well as to assimilate all the information acquired during the day.

But their sleep might not be as impenetrable as you might think, a new study suggests. Researchers in Hungary have discovered that canines can identify whether the sounds they hear while asleep are emitted by a fellow dog or a human.

They came to this conclusion after conducting an experiment with 13 pet dogs. The scientists recorded their brain electrical activity while they were asleep, in order to analyse the variations induced by sensory stimuli during the different sleep phases (slow wave and REM).

These stimuli were, in fact, vocalisations emitted by other dogs and humans. These included yelps, whines and growls, as well as coughs, laughs, sighs and yawns. To avoid startling the dogs awake, the researchers did not use “negative” vocalisations or sounds with sexual undertones.

This experimental protocol revealed that the dogs were able to determine whether the vocalisations they heard during sleep came from another dog or from a human, as well as their valence, i.e. the positive, negative or neutral nature of the emotion the sounds stirred in them.

“Dogs process information about emotional valence and species during sleep, actively reacting to the stimuli of their surroundings even in deep-sleep stages,” the researchers wrote in their paper, recently published in the journal “Scientific Reports”.

Other animals are able to do the same, including primates and mice. But this is the first time that dogs have been shown to be quite so sensitive to their environment while sleeping.

This finding must be treated with caution, however, due to the small number of dogs involved in the study and certain methodological limitations. Nonetheless, it improves our understanding of the brain function of these popular and cherished pets.