We often think of sleep as a chance to switch off from the outside world, leaving us blissfully ignorant of anything going on around us. But neuroscience research has shown this is a fantasy – we still monitor the environment and respond to particular sounds while we’re sleeping (at least in some stages of sleep) – a fact that will be unsurprising to anyone who has woken up after hearing someone say their name.
A better example is anyone who has woken up after hearing their infant moving around. Part of the sensitivity is making sure the kiddo is okay. She is snoring? Good. Cannot hear anything at all? Is she breathing? This assessment that everything is fine occurs while still sleep-addled, which to me is amazing. Most likely the movement means in about 10 minutes she is going to wake up.
Probably this was good for keeping us alive on the savannah. Lions are nocturnal (night awake) where we are diurnal (day awake). It would be good for us to be able to sense the noises of a predator possibly coming to hunt us.