Tefillin at Night:
Biblically one is obligated to wear Tefillin also at night. However the Sages prohibited one to wear Tefillin at night due to a decree that one may fall asleep while wearing the Tefillin. It is therefore forbidden to be placed on even after Alos Hashachar until the time that most people awaken, which is defined as the time of morning that it is light enough to recognize an acquaintance from a distance of 4 cubits.
 Admur ibid
Other opinions: Some Rishonim rule it is Biblically forbidden to wear Tefillin at night, as the evrse states “Yamim Yamima”. [Rambam Tefillin 3/10]
 The reason: It is Rabbinically forbidden to wear Tefillin at night because one may come to sleep in them and release bodily odors. [ibid as explained in 44/1]
 Admur 30/1; 30/5; M”B 30/5 “so long as it is before Tzeis”; Implication of P”M 30 A”A 7; Piskeiy Teshuvos 30/2 in name of Kesav Sofer 9 [did not find in Kesav Sofer]
 The reason: As the Sages prohibited one to wear Tefillin at night. [30/1] One cannot say that their decree does not apply in this case that one did not put on the Tefillin at all during the day, as even if it is prior to night and one simply Davened Maariv after Plag some Poskim rule one may not put on Tefillin. [Admur 30/5] Hence certainly in our case that it is after night, all would agree it is forbidden to put on Tefillin at night, even if he did not put it on that day. Furthermore, putting on the Tefillin at night would not even help for that day’s Mitzvah, as the night belongs to the next day and hence the current days Tefillin is certainly lost. [Piskeiy Teshuvos ibid in name of Kesav Sofer]