Ok I have this big drawer at the bottom of an armoire. It is full of old shorts, tank tops, sleepshirts, t-shirts, and a couple of pairs of pj pants. No matched sets, just bits and pieces. In fact, most of them were, at one point, work-out or summer clothes that got old and a little worn, and got moved from "daytime use" to "sleepwear". That's all I ever sleep in, winter or summer. My very fav thing to sleep in is a pair of knit shorts and a men's underwear-style tank top.
I do have a couple of nice silky nightgowns and pretty gown sets, but I don't sleep in them - I can't stand sleeping in long nightgowns, I HATE when things get tangled around my legs.