Just read on Pharyngula* about the giant squid eye. The squid and we share a common ancestor about 600 million years ago, back when the state-of-the-art eye was a patch of photoreceptive cells.
So, the fascinating thing about the differences in the ways our respective eyes work is that, put very simply and paraphrasing Dr. Myer's excellent summing-up, our photoreceptors are excited by an absence of light, while the cephalopods' eyes are excited by the presence of light.
So every time we close our eyes, our photoreceptors start partying like it's 1999.
I have no idea why that should be, but it obviously doesn't bother us when we sleep.
*Warning! Science content!