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44:1. At all times when one is wearing Tefillin on his head or arm, it is prohibited even to take (1) a nap, unless one places a handkerchief on top of them and (2) his wife is not with him - then (3) he can (4) nap while wearing them. How must he do this? By placing his head (5) between his knees in a sitting position, and falling asleep that way. If his Tefillin are wrapped in his hand, (6) it is even permissible to fall fully asleep - but if he is holding them in his hand and they are not wrapped, (7) even napping is forbidden. {Rama: And this is only if he is holding them without their case, but if he has them in their case (8) any sleep is permitted.}
MB 1: A nap - Because we made an ordinance lest he pass wind while wearing them; and by placing a handkerchief over them, he will remember that he is wearing Tefillin [even while napping] and will not pass wind.
MB 2: His wife is not with him - But if she is with him it is forbidden, lest he have family relations while wearing them.
MB 3: He can nap - One is only considered to have taken his mind off of them when he is making fun and lightheaded, but when he is busy with his work or trade, and he is not concentrating completely upon them, this is still not considered taking his mind off of them (unless he involves his mind so much with the needs of his body, that his heart turns away from fear of heaven because of his involvement). So too when one sleeps he turns away from the nothingness of this world. Nonetheless, he is performing the commandment at its best when he keeps his mind upon his Tefillin constantly, in order that he does not take his mind off of them to be drawn and to think impure thoughts - and because of this a person is obligated to frequently touch and move his Tefillin, also in order that he not take his mind off of them - except when he is praying or studying, at which times he need not give his mind over to his Tefillin.
MB 4: Nap - There is no time limit on this; some say there is a limit of the time it takes to walk 100 footsteps, which is two thirds of an hour; so rules the Gr"a in his explanations.
MB 5: Between his knees - Because if not, we fear that he may fall completely asleep.
MB 6: It is even permissible - because in this case we do not fear that he may pass wind [while wearing them], as he is not wearing them.
Siman 44
. The prohibition on Sleeping while wearing Tefillin.