Siman 44

Since tefillin require a clean body so that one does not flatulate in them, it is therefore forbidden to sleep in them, for in sleep one is close to flatulating (Shabbat 49a). And we learn in Sukkah (26a): One Tanna taught: A person may sleep in tefillin a brief sleep but not a fixed sleep. And another Tanna taught: Both a fixed and a brief sleep. And another Tanna taught: Neither a fixed nor a brief sleep? It is not a difficulty: That which was taught that even a brief sleep is forbidden—this is when he holds them in his hand, for we fear lest they fall from his hand. And that which was taught that a brief sleep is permitted and a fixed sleep is forbidden—when they are placed on his head, for in a brief sleep we do not fear flatulation and in a fixed sleep we do fear it. And that which was taught that even a fixed sleep is permitted—where he spreads a cloth over them. And Rashi explained that he placed them near his head and spread a cloth, see there. And according to this, that which was taught in Berakhot (23b): A person should not hold tefillin in his hand and pray..., and he should not sleep in them neither a fixed sleep nor a brief sleep, see there—the reason is lest they fall from his hand. (And it is a wonder regarding Rashi who explained there: lest he flatulate, see there. And it requires great study.)

But the Rambam wrote in the fourth chapter, law 15: Tefillin require a clean body… therefore it is forbidden to sleep in them, neither a permanent sleep nor a temporary sleep. However, if he spread a cloth over them and a woman was not with him—he may sleep in them a temporary sleep. And how does he do this? He places his head between his knees while he is sitting and sleeps. If his tefillin were wrapped in his hand—it is permitted to sleep in them even a permanent sleep. Thus far his words. And he explains that that which they said "that he spreads a cloth over them," this is its explanation: that they are placed on his head, and without a cloth it is forbidden even for a temporary sleep, for we fear lest he sink into sleep even when he places his head between his knees. But when he spreads a cloth over them—he will certainly awaken from his sleep in a short time. But there is no reason or logic in this, obviously, and therefore it must be said that without a cloth we fear he will forget that he is wearing tefillin, and by means of the cloth he will remember (Kessef Mishneh) and will not come to flatulence. And therefore it is permitted for a temporary sleep and not for a permanent sleep. And that which was taught in a Baraita that even a permanent sleep is permitted—this is when they are wrapped in his hand not on the place of tefillin, and they are wrapped tightly so they will not fall. And why did they not forbid it because of distraction of the mind when wearing them on his head? They wrote the reason that distraction of the mind is nothing other than levity and lightheadedness, as I wrote in siman 28. And we further explained there that during the time of sleep the prohibition of distraction of the mind is not applicable, for he is not in the world then at all, see there. And so it is implied in the Levush, see there.

Our teacher, the Beit Yosef, transcribed the words of the Rambam, and added to them that if he holds them in his hand and they are not wrapped in his hand – it is forbidden to sleep in them even a brief sleep. Thus far his words. And the Rambam did not see fit to mention this, as it is a simple matter. Since he wrote that wrapped is permitted, you automatically hear from this that holding without wrapping is completely forbidden. And the primary concern is self-evident: lest they fall. Our teacher, the Rema, wrote that this is specifically when he holds them without their case. But in their case – it is permitted in any manner. Thus far his words. That is to say: we are not concerned lest they fall to the ground, for even if they fall there is no prohibition since they are in their case. According to this, also regarding that which the world is accustomed that when through some cause the tefillin fall to the ground, they fast or give something to tzedakah, when they fell in their case – they do not require anything. (Magen Avraham subsection 5. And I wonder at what he wrote in subsection 3 regarding the measure of a brief sleep on the Shulchan Aruch. For he follows the view of the Rambam that placing his head between his knees constitutes a brief sleep, as the Rambam ruled like Rabbi Yochanan in Sukkah there. And he should have written this on the Tur, for the Tur did not mention the measure of a brief sleep at all. And as the Taz wrote in subsection 1, see there.)