Whereas a dog’s sense of smell is almost fully functional while they sleep, a cat’s hearing is working almost at full functionality while they sleep; especially for higher frequency sounds. Cat’s also have the ability, as far we know (without being able to actually ask them), to filter sounds they don’t care about. Sounds like the dishwasher going or people talking.
But if something gets through the filter they are awake in an instant. Once they determine that nothing is amiss they go back to sleep within seconds. Don’t you wish you could do that?
While they are sleeping they rely almost totally on their hearing to keep them alive should something go wrong in the ‘outside’ world, and their ability to go from sleep to instant action.
Here is a shot of Sansa sleeping. Notice the ears turned forward to pick up the slightest unexpected sounds as she sleeps.
Apparently they can also detect movement on their whiskers while asleep but I have touched Sansa’s whiskers when she is sleeping and nothing happened. Maybe her whisker sensing mechanism is faulty.
Sadly for us humans our sense of smell almost totally shuts down while we sleep and our hearing drops to about 20 percent; even less in deep sleep.