Comment by NoMoreNicksLeft

I don't know if I can buy this explanation. Sleep is dangerous (and not just to night drivers). You're basically in a several-hours-long coma where a smilodon can come along and eat you without any trouble. So long as cells have more than one mitochondria each, staging them so they don't all need sleep simultaneously seems like a total no-brainer, and doesn't require any difficult-to-manage circumstances that leave you unconscious as predator snacks. This is a big deal, there's more than enough evolutionary pressure for sleep to have been selected out of the genome hundreds of millions of years ago.