When we go into deep sleep (stages 3 and 4), we lose all connection to the external world (otherwise known as “sensation”). One sense that does persist is pain, but absent a painful stimulus, we don’t wake up. Video Rambler points out an online video showing a cat that seems to be in a very deep stage 4 sleep. You can manipulate its limbs, and even shake it a little, before it takes a violent shaking to wake up.
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