If one knows that he will not be able to put on Tefillin the entire day, even for a moment, may he put them on in the early morning, prior to Mi Sheyakir?^[1]

If one knows he will not be able to put on Tefillin for even one moment that day, as can occur by one who works throughout all the daylight hours, then he may place them on upon awakening, even if it is prior to Alos.^[2] He is permitted to recite a blessing upon wearing them at this time if he will be unable to wear them at all after Mi Sheyakir, and throughout the day.^[3] Nevertheless, it is best for a Torah scholar not to say a blessing upon wearing it.^[4]

[1] Igros Moshe 1:10; Piskeiy Teshuvos 30:1 in name of Poskim; Divrei Efraim in Emek Habracha 2 that so he ruled in times of the Holocaust in the ghettos and camps; Heard from Rabbi Mingal that when he was nine years of age in Auschwitz all the men lined up in middle of the night and put on his father’s Tefillin with a blessing.

[2] The reason: As we rule that Biblically the Mitzvah of Tefillin applies even at night and it is only due to worry of falling asleep that we decree against wearing Tefillin at night. Hence, if one already woke up from his sleep and will not be able to put on the Tefillin at any other time, then one may wear it then as there is no worry that he will fall asleep. [Igros Moshe ibid; Divrei Efraim ibid] We have no proof to say that the Sages decreed against wearing Tefillin at night even if this will uproot the Mitzvah completely. [Divrei Efraim ibid]

[3] Poskim ibid

The reason: As Rabbeinu Peretz rules that a blessing may be said at night by a traveler, and although we do not rule like his opinion, nevertheless perhaps in this case that he will not be able to say a blessing later on all would agree that he can say a blessing at this time. [Igros Moshe ibid]

[4] Igros Moshe ibid