We all need sleep. Most of us love nothing more than sliding between soft sheets at the end of a long day, flipping the pillow over to the cool side, and heading to dreamland.

But sleep is largely a mystery, even though science has devoted quite a few studies to it and why we need it so badly. What happens when we sleep is a little less mysterious–at least, from a physical perspective. Here are 16 things you might not know about sleeping, dreaming, and how common it is for couples to still sleep in separate beds.