I hate flat sheets anywhere on or in the bed. I overheat so easily under a doona already, I don't need another layer to add to my sleep discomfort.

And flat sheet on fitted sheet sucks too because I just end up tangled in it, leaving me all twisted and contorted by morning.

I prefer the flat sheet under the fitted sheet; makes the bed feel more comfy. Doona for winter, and a blanket for summer is enough for me lol