I spent over three weeks fighting a cold that refused to die. As a result, I spent several days on my couch, wearing sweatpants. And I have two rules for wearing sweats: 1) unless you are on your way to or from the hospital/doctor, the sweatpants do not leave the house, and 2) make sure they look nice and lounge-y, not schlumpy and frumpy.
These are my favorite. I have them in every color.