When one sleeps during the day, does the blanket or sheet need to have tzitzit because it is a four cornered garmet and if not - why not?

It does not need Tzitzit (Shulchan Aruch Orach Chayim 18, 2). Some say the reason is because the main use of the blanket and sheet is at night; others explain that only something warn as a garment needs Tzitzit (Mishna Berura 18, 8)