The Talmud records a prohibition of sleeping with tefillin on, apparently connected to two concerns (see Tosafot, Shabbat 49a, s’v sh’lo): the inability to maintain complete bodily cleanliness, and the prohibition of distracting one’s self (heseich ha-da’at) from the tefillin (see Shabbat 12a; Yoma 7b; Menachot 36).

However, the Talmud (Sukkah 26a) does allow “sheinat arai” (such as a brief nap) while wearing tefillin. Such sleeping would seem to consist of “heseich ha-da’at”. In response, the Rosh (Berakhot ch. 3, 28) suggests, in the name of Rabbeinu Yonah, that heseich ha-da’at only occurs when one’s mind is turned to frivolity; however, for one who is involved with other matters in a serious fashion, or asleep and thus removed from trivialities, heseich ha-da’at is not a concern.

As noted by the Sha’agat Aryeh (#39; see also #s 38 and 40), the Rambam and the Ramban seem to feel differently, both concerned with heseich ha-da’at even when frivolity is not present. In discussing the distraction of one in an unsettled state (see Mishneh Torah, Tefillin 4:13) or in a period of mourning (see the Ramban’s Torat HaAdam), both maintain that tefillin may not be worn if focus cannot be obtained. Thus, the implication is that even when frivolity is not an issue, heseich ha-da’at may be. Accordingly, the license for sheinat arai with tefillin on is difficult to understand.

R. Yitzchak Blazer (Resp. Pri Yitzchak, I, 5) notes that the Magen Avraham (O.C. 308:11) rules that according to the opinion that Shabbat is not a time when tefillin are worn, one who wears tefillin on Shabbat need not maintain constant awareness of their presence. Thus, the concern of heseich ha-da’at is apparently in effect only when the mitzvah of tefillin is being fulfilled, and not when the tefillin are being worn without a mitzvah. One who is asleep or unconscious is unable to perform mitzvot; hence, there is at that time no issue of heseich ha-da’at.

The S’ridei Eish (Resp. I, O.C. 4; note also #5) adds to this approach by noting that tefillin are termed “pe’air” (glory). Therefore, in the Rambam’s view, the mitzvah of tefillin requires an awareness of that pe’air, and cannot be performed without it. While asleep, though, one is exempt from the entire mitzvah of tefillin, and thus merely wearing them would not impose a prohibition of heseich ha-da’at. R. Yonah, however, understands heseich ha-da’at to be a function of disrespecting tefillin, a concern not present when the preoccupation is not of a frivolous nature.