Ha! This depends on a variety of factors.
Usually in an oversized tee shirt. Pretty comfy. And long enough to not offend anyone if you have to dash to the bathroom or kitchen.
When freezing cold in middle of winter I have been known to wear thermals/trackpants/jersey/fluffy socks. All together. Usually wake up 100 degrees too hot though.
When drunk, sometimes passed out still in my dress. Sometimes I manage to get the dress off though. Classy.
In summer, well summer’s hot…
I don’t even own real pyjamas! I feel like if I bought some (Peter Alexander darling) they’d be too pretty to wear in bed and I’d just end up wearing them around the house.