For those of us who choose memory foam, we do so because the way it molds to our body. The problem that I've found is that when you put a sheet on it, the "give" changes and is sometimes eliminated almost completely.
My wife and I use cheap, Jersey Knit sheets from Wal Mart. They seem to be the most comfortable, warm sheets we could find. Plus, they don't shift around as much when I toss and turn. I have a hard time sleeping (nightmares from PTSD) so I move around a lot. The sheets stay in place pretty well.
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