For Christmas I received a pair of flannel pajama bottoms. I've but them to great use, but not for sleeping. When my legs get cold I slide them over my jeans. Sort of like wearing a flannel shirt over a regular shirt.
That's clever. In Austin, TX, where I live, some people even go out in public in their PJs. It was funny to see a grown man at Whole Foods wearing flannel pajama bottoms with footballs printed on them. We are very casual here.
In the winter, I always wear black leggings under all my slacks and jeans. I don't know if they sell these in the men's department but they probably have thermal underwear. Whatever it takes to stay warm, right?
I have long underwear that I can wear. But sometimes they make me sweat too much, so I don't put them on. Other times I need them plus something else. So it is convenient to have the flannel ones because they are quick and easy.
I bought a pair of Wrangler "thermal" jeans. These are made with demim that's a little heavier than usual, and lined with what they call "fleece," and what I call "fuzzy flannel." Anyway, these are working out better than my other solutions. They don't feel too hot indoors. And they seem to do the job outdoors, but this needs more testing. Style-wise, they look awful. But I'm old so I'm allowed to dress without style.