Since we’re all posting opinions: flat sheets go straight into the fabric bin, not on beds*. My comfort level is just the duvet inside a duvet cover and nothing else, unless it’s too cold, in which case additional blankets go on top of the duvet cover. Sheets are too thin and prone to twisting around your body during the night, thereby waking you because you’re all tangled up. Duvets inside duvet covers don’t do that and so they are superior.

* Except for the guest bed, because a lot of people are used to having a flat sheet, and I don’t want to put our guests out by having a different bedclothes setup than what they’re used to.