There are varied opinions among the Tannaim regarding if the mitzva of tefillin fundamentally exists at night.[1] According to most Rishonim,[2] the mitzva of tefillin does apply at night on a Biblical level. Nevertheless, one is Rabbinically forbidden to put tefillin on at night, because of the concern that he may fall asleep and then pass gas while wearing the tefillin, which would be degrading to them.[3] (In certain circumstances, such as if the tefillin are still on from daytime[4] or if one needs to wear the tefillin so as to protect them,[5] the matter is somewhat complicated.) Although the halachic day starts at alot hashachar, approximately 72-90 minutes before sunrise, tefillin should not be put on until “misheyakir” (when it is light enough to recognize an acquaintance at the distance of four amot),[6] approximately 50 minutes before sunrise.
A baraita[7] teaches that if one will be on the road from before the time of tefillin, and he is concerned that if he does not wear them before he begins his travel, they are more likely to get lost, he may don them early, but he should delay the beracha(ot)[8] until the proper time arrives.[9] The Shulchan Aruch[10] paskens according to this baraita and explains that the ordinary concern of falling asleep at night does not apply in this case, because this person has already gotten up for the day. The Mishna Berura[11] assumes that this traveler may put on tefillin even before alot hashachar.
One crucial question relates to the nature of this special permission to put on tefillin early. Is it an indication that one is, under special circumstances, able to fulfill the mitzva before its normal time (which is your desire)? Or does it simply reflect that one is allowed to wear tefillin in order to protect them, without fulfilling the mitzva at that time?
The fact that one should not recite the beracha until later could be an indication that he does not yet fulfill a mitzva. In fact, Rabbeinu Peretz[12] maintains that the baraita is in accordance with the view that the mitzva of tefillin does not apply at night; according to our ruling that it does, the traveler should make the beracha right away. According to Rabbeinu Peretz, this individual is clearly performing a mitzva. However, the Shulchan Aruch[13] rules that one should not recite the beracha then, despite the fact that he maintains that the Torah-level mitzva applies at night. The Sha’agat Aryeh[14] posits that although one fulfills the Torah-level mitzva of tefillin at that time, since the Rabbis generally required waiting until misheyakir, the berachot were not instituted for unusual circumstances when it is permitted to don them earlier. Along similar lines, the Eliya Rabba[15] writes that if one mistakenly makes the beracha before daytime, he should not repeat it at the right time, because the beracha was valid due to the Torah-level fulfillment. Rabbi Akiva Eiger[16] agrees that one should not repeat the beracha in that case, but his rationale is different. Since it is possible that Rabbeinu Peretz is correct and the beracha was appropriate l’chatchila, one should not make a new beracha, as making a new beracha would require confidence that the first beracha was invalid.[17]
In the final analysis, while some disagree,[18] we assume there is benefit to putting the tefillin on before alot hashachar if the alternative is not putting them on at all. This is the recommendation of both Rav Moshe Feinstein[19] and Rav Moshe Sternbuch,[20] each of whom adds his own practical perspective on the matter. Rav Feinstein writes that if the person will not take the opportunity of putting on tefillin seriously if he is not instructed to make a beracha, we can rely on Rabbeinu Peretz’s view and have him make the beracha even before the proper time. Rav Sternbuch comments that he believes it is rare for a person to have no opportunity to put on tefillin for a few seconds during the day; he suspects that the issue is more often embarrassment to put them on at the workplace, which he does not consider justified in our times.
As we have seen, permission for the early-waking traveler to put on tefillin before dawn is predicated on the assumption that he will not fall asleep under those circumstances. Therefore, if one is not walking or riding but is being drawn in a vehicle, he may not wear tefillin before dawn.[21] Similarly, and relevant to your question, if a doctor is in the midst of a long shift in which he is able/expected to catch power naps, the leniency does not apply, as we must be concerned that he might fall asleep while wearing the tefillin.