Mishnah Berurah — משנה ברורה

When he gets up - Meaning, when he gets up and goes out from under the cover of his sheets/blanket where he was lying naked when he removed his [night] shirt/tunic, now he will be covered immediately [when he inserts his head and arms into the day shirt/tunic under the sheets] because his shirt/tunic will fall over his whole body on it's own. See further below in siman 239, se'if 2 regarding removing one's shirt/tunic. One who places his clothing underneath his head will forget his learning, and perhaps if one places something else in between his head and his clothing, there is no problem. Similarly, one should be careful not to put on two articles of clothing at once, because it causes forgetfulness. One who reads the inscription on a gravestone or who gazes upon a corpse will forget his learning. See further in Pri Megadim for things which restore a person's learning.