Birds resting on a beach or a wire often look calm, almost detached from what is happening around them. But that stillness can be misleading. For many species, sleep is not a full shutdown. One eye ...

Humans do very little when we sleep. This might not seem like a big deal, until you look to the dolphins as they sleep and swim with the pod. Or fur seals, who can sleep while watching out for sharks.

Have you heard the saying about “sleeping with one eye open”? It’s a bit of metaphoric advice about staying vigilant—and a way to describe very light restless sleep. But sleeping with eyes open is ...