This is shocking to me!!
I never knew anyone who didn't use a top sheet, and used to think if someone didn't must be impoverished or didn't have good access to laundry, LOL!!
I was raised mattress pad, bottom sheet, top sheet, blanket (could be down) and bedspread. This was a "proper" bed. Maybe it's a Midwest thing, I don't know.
I would never would use a duvet, too much work. Now I just use top and bottom sheet for sleeping mostly because I'm always hot. In the winter, top and bottom sheet and a light goose down blanket. I find flannel sheets comfy to sleep on, even in the summer.