Your ears are always turned on! During light sleep, our brains can still process sounds and words from the environment.
In deeper sleep, our brains process sounds less, but can still alert us when there is an unfamiliar sound in our environment.* For example,
the hum of an A/C turning on or highway noise is something we may get used to during sleep, but sounds like a child getting out of bed or an
emergency siren nearby can wake us from deep sleep.
*Hayat, H., Marmelshtein, A., Krom, A.J. et al. Reduced neural feedback signaling despite robust neuron and gamma auditory responses during human
sleep. Nat Neurosci 25, 935-943 (2022).
https://doi.org/10.1038/s41593-022-01107-4