Default Mode Network

Refers to the brain regions most active when the mind is at wakeful rest; involved in self-reflection, daydreaming, memory, and imagining the future.

It’s a bunch of structures in your brain that switch on when you don’t have anything else to think about. So, you forget your phone and you’re sitting at a light, for example. That’s when the default mode network goes on, and generally, it’s an unpleasant feeling.

source: Harvard Video - You Need to be Bored, Here’s Why