I once came home from work to find my Zoe in a very deep sleep. Her eyes were only half closed, however, and she didn't initially respond to my voice. Then I started to gently clap my hands as I slowly approached her cage. Still no movement. At that point, my heart was in my throat, thinking that some mystery illness had claimed her her life. So I was especially shocked (and gulped down my heart) when she suddenly lifted her head, staring at me as if to say, "Wha's goin' on?"
Since that experience, I've a newfound respect for the seriousness of the bunny flop and how thoroughly they can sleep (for a prey species, that is).
Jenk