Is this a universal constant with no solution? A sheet will crease and wrinkle under you? Even with a fitted sheet? Even an elastic material? Even with straps under the mattress to try a pull it tight? Is there really no answer????? https://toot.me.uk/system/media_attachments/files/112/129/436/914/000/837/original/4f12c9585e638c11.jpeg
nailgun. But your sheet seems like it might be too deep for your mattress as I don't tend to have this problem with our fitted sheets (particularly after they've been washed a few dozen times and have shrunk a little). ...or you flail around in your sleep *a lot*?