Yoma 69A

וסיפא איצטריכא ליה: פושטין ומקפלין ומניחין תחת ראשיהם.

That Mishnah’s teaching highlighting the prohibition to sleep in priestly vestments is needed for the latter clause of that Mishnah, which states: They remove their priestly vestments and fold them and place them under their heads. Since they are allowed to sleep on them, it must be emphasized that they may not sleep while wearing them.

פושטין ומקפלין ומניחין אותן תחת ראשיהן, שמעת מינה בגדי כהונה ניתנו ליהנות בהן! אמר רב פפא: לא תימא תחת ראשיהן, אלא אימא כנגד ראשיהן. אמר רב משרשיא: שמעת מינה תפילין מן הצד שפיר דמי.

The Talmud considers resolving the dilemma from the latter clause: They remove their priestly vestments and fold them and place them under their heads. The Talmud suggests: Learn from this that it is permitted to derive benefit from priestly vestments. Rav Pappa said: Do not say that the Mishnah means they may actually place the vestments under their heads as a pillow; rather, say that the Mishnah permits the vestments to be placed only next to their heads. Rav Mesharshiyya said: Given this understanding of that Mishnah, one can learn from here that one who places tefillin to the side of his head when he sleeps has done well; there is no concern that he will turn over in his sleep and lie upon them.

הכי נמי מסתברא דכנגד ראשיהן, דאי סלקא דעתך תחת ראשיהן, ותיפוק לי משום כלאים ⁠— דהא איכא אבנט. ונהי נמי דניתנו ליהנות בהן ⁠— הא מתהני מכלאים!

So too, it is reasonable to say that the Mishnah permits the vestments to be placed only next to their heads and not under their heads; as, if it could enter your mind to say that the Mishnah permits the vestments to be placed under their heads, and I would derive that it is prohibited due to the fact the priestly vestments contain a forbidden mixture of diverse kinds, as among them there is the belt, which is woven from a mixture of wool and linen. And even if it is assumed that it is permitted to derive benefit from priestly vestments, it would still be prohibited to lie upon them because by doing so the priests would be deriving benefit from a garment made of diverse kinds.