Why do we not hear sounds while sleeping?

A: Sleep does not mean the brain is inactive and we do hear sounds while sleeping. Sleep is a recurrent and healthy state of inertia and reduced responsiveness found among higher vertebrates. It is controlled by the brain and is associated with characteristic electrical rhythms in the brain.

The body induces this state when the central core of the brainstem situated below the cerebral hemisphere is stimulated. Likewise, stimulating the reticular formation (a portion of the central nervous system which consists of small islands of grey matter separated by fine nerve fibres bundles running in every direction) by messages from the cerebral cortex can awaken sleeping persons.

Neurons (nerve cells) in REM sleep are as alive as they are in wakefulness. Mental capacity also does not decrease. Incoming sounds are also subjected to ceaseless scrutiny. But the brain ignores the unimportant ones while the important ones, even if feeble, lead to arousal. The Arousal thresholds are variable and are a function of the meaningfulness of the stimuli. When a stimulus has no significance to the sleeper, the thresholds can be rather high. So irrelevant stimuli are actively shut out during REM sleep.

Behaviourally, researchers have established that motor responses can be evoked in all stages of sleep, but it is difficult to demonstrate that new responses can be acquired during sleep.