Flannel sheets on real cold weeks in winter, otherwise cotton year round. I have 3 blankets weights (winter, spring/fall, & summer) that I rotate thru during the year. The summer weight one started life as a hospital blanket before I “adopted” it. I’ve had it so long that you can almost see thru it. I like something covering me, but as light as possible.
Naked 
Hmmm.I have down comforters in which the down is over 60 years old to my knowledge, and I wash them yearly as did my mother and grandmother before her.
So Cal here. A fitted sheet, a top sheet, and an IKEA cool comforter. It’s fuzzyish but not scratchy, just provides a bit of comfort if there is a cool breeze, but will not make you too warm.
Pacific Northwest. Beginning in May, usually, soft cotton sheets and a knitted cotton cover. If it gets too cold, a fleece lap throw on top. We only cool the house to 72 at night. Easy enough to run a fan or air filter.
If it ain’t broke, don’t fix it. 
I got the 2 year figure from a neighbor whose family owns a chain of dry cleaners. This is the interval they recommend to their customers. I figured that was an authoritative enough source for purposes of this board.
It’s entirely possible that modern down items aren’t made nearly as well as the ones from 60 years ago, and can’t take more frequent cleanings.
I just wash them in the washing machine. The down itself is well over 60 or 70 years old, but the fabric encasing them has been replaced several times. My grandmother picked the down off her own geese.
How do you dry them? On the line? In the dryer? I would think it would take a while to dry a well-stuffed down item.
Cotton flannel in the winter.
Cotton in the summer.
Not especially, aside from the fact that you absolutely can’t use bleach on linen, even if it’s white linen. It’ll yellow the fabric. And if you fold it in sharp creases the creases will stick in the fabric permanently rather quickly (so rolling your sheets for storage is better).
Other than that, you can wash linen as hot or as cold as you like, line dry preferred, tumble dry low if necessary. It wrinkles easily but has a nice bouncy quality to it and the wrinkles smooth out during use. Fabric gets softer with each wash. Supposed to hold up for ages though (I’ve only had them a year so far).
Cotton sheets, and a relatively thin down comforter all year round. But I live in The Land that Seasons Forgot. In the winter we close the windows if it’s a particularly cold night and we remember to.