In a properly functioning body, sleep helps the brain process your emotions and memories from the day. You wake up well-rested with enough mental space to both create and log new memories and work through the experiences of your day.

Sleep deprivation, on the other hand, is like falling into an icy river: “The body shuts down circulation to the appendages and tries to keep the core warm. It goes into survival mode,” says W. Christopher Winter, a neurologist based in Charlottesville, VA and the author of The Sleep Solution. When you’re not sleeping well, “your brain’s ability to do things gets whittled down to: find food, urinate, get through the day,” he says.

That means superfluous activities—like conversations with your partner, social outings or remembering to pick up the dry cleaning—go out the window.