Sleep remains one of biology's most confounding mysteries. We are just beginning to understand how it evolved, but defining this strange state of consciousness is incredibly difficult — especially because there are so many ways to experience it.
Humans obviously love sleep. Like all mammals, we literally can't live without it. But when did our capacity for, and dependence upon, sleep evolve? On a related note: how common a phenomena is it, really? Can we say, for example, whether organisms like bacteria sleep in the same way that humans do?