Mishnah Berurah — משנה ברורה

ציצית to them - Since their main use is at night and [therefore] it's called a "nighttime garment". But according to this, it would only be exempt [from ציצית] according to the Rosh; but according to the Rambam [who holds] that a nighttime garment is obligated [in ציצית] during the day, it (ie. the bedsheet/blanket) would be obligated. And there are those who say another reason to exempt it: it's because the Torah only obligated [a garment] that's worn as clothing or that one wraps [himself] in it, but not something that one covers himself with. And there are those who argue on this and hold that even if one spreads out a 4 cornered garment underneath himself, and certainly if he covers himself with [a 4 cornered garment]. Therefore, the Magen Avraham decided that one should make one of the corners rounded and [therefore] would fulfill all opinions. But it's inferred from his words that one only needs to be stringent regarding this with a wool bedsheet/blanket, but not with one of flax or some other material. And regarding the reasoning for this, see in Pri Megadim....