Electrophysiological methods of studying the nervous system have opened up new opportunities for investigations of sleep. Striking changes in the pattern of EEG during the transition from wakefulness to sleep made it’s recording a mandatory element of any somnological research. It was also found that the frequency of neuronal firing in the cerebral cortex during sleep does not decrease, but can significantly exceed the average level of the cortical activity during wakefulness. But it remained the main mystery of sleep, what was associated with this high activity of cortical neurons at a time when the thresholds of sensory perception raised and the propagation of signals from the outside world and from the own body towards the cerebral cortex were almost blocked. The resolution of this paradoxical situation was suggested by the visceral theory of sleep. This theory was based on the assumption that during sleep interoceptive afferentation from all visceral systems of the body arrives for analysis into the cerebral cortex. This article offers an overview of the studies performed for direct experimental verification of non-trivial predictions of this theory.