With all of its variables and mysteries, sleep (like the weather) provides for an endless source for small talk. How did you sleep, and did you get enough? Was it hard to fall asleep, or hard to wake up? Did you dream, and what about?

It’s unclear why, but people need sleep to survive. Insects, fish, and animals need it, too, but some less and others more, some hibernating for months. Sometimes sleep eludes people, no matter how tired they are, and other times the well-rested can’t stop themselves from nodding off throughout the day. People dream, but often can’t remember what about, or they dream the same thing over and over and over again.

Sleep is fascinating, and not just to regular people, but also to scientists who can now use technology like fMRI and modern biochemical… More…