It just depends with mine. If Charley's out of town (rarely) Bimmer only sleeps lightly and the slightest noise puts him silently on his feet. I've noticed him sleeping sometimes - or at least shamming sleep - with his eyes slitted open the tiniest bit.
If I'm in the kitchen, Shiva will position herself on the sofa where she can see me and sleep with one eye open - just in case there might be a floor related food incident. Kharma feigns sleep quite often . . . they're a deceitful pair; always looking for an opportunity to take advantage of, lol.