Would a sleeping bag need tzitzit because you get inside of it, making it kind of a garment? What about shatnez? Since you get into the sleeping bag like a type of garment, does it need to not be wool and linen mixed together?
Would a sleeping bag (th warm weather ones) need tzitzit because you get inside of it, making it kind of a garment?
A sleeping of this type of sleeping bad has no four corners. warm weather sleeping bag
Despite the rectangular aspect it is as a flattened cylinder (despite the possibility to open it with the zipper, the use is when the zipper is closed).
But even a rectangular blanket or bed sheet, are exempted from the requirement of tzitzit. because they are night garments (as mentioned by @Ploni Almoni from SA). See Menachot 43a:
It was taught: That ye may look upon it: this excludes a night garment.
What about shaatnez? Since you get into the sleeping bag like a type of garment, does it need to not be wool and linen mixed together?
See Mishna Kil'ayim, 9, 2,:
הַכָּרִים וְהַכְּסָתוֹת אֵין בָּהֶם מִשּׁוּם כִּלְאַיִם, On the first glance, the Mishna seems to permit: Cushions and blankets has no problem of Kil'aym. But Bartenura explains the real meaning of this statement:
לְפִי שֶׁהֵן עֲשׂוּיִין לִשְׁכִיבָה, וְהַתּוֹרָה אָמְרָה (וַיִּקְרָא יט) לֹא יַעֲלֶה עָלֶיךָ, אֲבָל אַתָּה מֻתָּר לְהַצִּיעוֹ תַּחְתֶּיךָ. וְהַנֵּי מִילֵי כְּשֶׁהֵן קָשִׁים וְרֵקָנִים וּנְתוּנִים עַל גַּבֵּי אִצְטַבָּא,
They are hard and not convex. He adds they don't touch the skin, so cannot wrap by a thread a small part of the body.
__In summary the sleeping bad cover the body on top and on bottom, is flexible and concave, and sometimes touch the skin. Each of these details is sufficient to make it susceptible to be prohibited by shaatnez
(Sources provided from Sefaria)
סְדִינִים, אַף עַל פִּי שֶׁאָדָם יָשֵׁן בָּהֶם בַּבֹּקֶר, אֵין מְטִילִים בָּהֶם צִיצִית.
Bedsheets, even though one sleeps in them in the morning, should not have tzitzis attached.
The Magen Avraham (see over there) brings down that there opinions that differ upon this idea and recommends that one should round one of the corners of the sheet so as to not obligate it in tzitzis according to everyone.
Hope this helps (partially) .