“Something nameless hums us into sleep,” the poet Mark Strand wrote in his beautiful ode to dreams. But what is that nameless something, exactly? By now, scientists know that sleep obeys our complex internal clocks, affects our every waking moment, and even tames our negative emotions. But even as they’re beginning to shed light on what happens while we sleep, they don’t yet know why we evolved to sleep in the first place.

In this fascinating short video, PBS’s Joe Hanson explores the mysteries of sleep, synthesizing science from David Randall’s excellent Dreamland: Adventures in the Strange Science of Sleep (public library) and other scientific curiosities, from how tiny ocean-dwelling worms explain our brains’ response to daylight and darkness to Edison’s power-napping strategy for success.

Sleep might be the single most important behavior that humans and other animals experience.

Complement with the chronobiology of why you’re so tired, the science of sleep and the teenage brain, and the relationship between dreaming and depression, then revisit this visualization of famous writers’ sleep habits vs. creative output.