Humans need sleep. Our bodies don’t care how busy or worried or uncomfortable we are: Eventually, we must succumb. But how can anyone rest while perched on a ledge thousands of feet off the ground or wedged into the prow of a heaving boat?

Amazingly, these explorers, photographers, and adventurers find a way.

European philosophers used to think of sleep as a liminal state, a moment in which a person’s otherwise active sensory system was in suspension. Sleep was a time to restore a person’s finite “sensorial powers” and recharge them for further exploration of the waking world.

However, modern sleep science tells us that the story is more complex, and that it’s not a passive activity. Once we slip into