The Gemara in Masechet Horayot (13b) lists several behaviors that can cause a person to forget the Torah he has learned. In our day and age, when our memories are very limited, it is especially important for us to avoid these behaviors so that we do not further handicap our memories.
One of these behaviors is "placing one’s Kelim underneath his head," which Rashi explains as referring to one’s clothing. If one takes a garment, places it under his head like a pillow and sleeps on it, he runs the risk of losing his power of retention. This is cited as Halacha by the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933), in Siman 2 (Se’if Katan 2), and the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Pinhas (Shana Sheniya, 17). Thus, one must ensure not to rest his head on any garment, including shoes.
There are, however, a number of exceptions to this rule. The Poskim write that this danger arises only if one rests his head directly on a garment. If, however, there is something in between one’s head and the garment, then resting on the garment does not pose any risk. Thus, for example, if a person has a suitcase with him at the airport, and he wants to put his head down on his suitcase to rest while waiting his flight, he may do so, even though there are clothes inside the suitcase. Since he does not rest his head directly on the clothing, there is no risk entailed. Furthermore, a person wearing a hooded sweatshirt may lean his head back to rest even though it rests directly on the hood. Any garment that a person is currently wearing is considered part of the person himself, and therefore he is not considered as resting his head on a garment. This is the ruling of the work Sefer Ha’zikaron.
Hacham Ovadia Yosef, in his work Yabia Omer, cites different opinions as to whether the risk mentioned in the Gemara applies even if a person engages in these behaviors infrequently. According to one view, these behaviors can affect a person’s memory only if he engages in them regularly. If one sleeps on garments on rare occasions, according to this view, he does not run the risk of diminished retention. Others, however, maintain that even doing so once can threaten one’s memory. As mentioned, in our generation we in any event have deficient memories, and it is therefore worthwhile to exercise caution in this regard and avoid resting one’s head on a garment even on rare occasions.
Summary: The Gemara lists a number of behaviors that have an adverse effect on one’s memory, including resting one’s head directly on a garment. This does not apply if there is something in between one’s head and the garment, or if one rests his head on part of a garment he is currently wearing, such as the hood of a sweatshirt.
