In the morning brachos of Krias Shema, we begin by praising G-d who”forms light and creates darkness.” When we recite this, there is a custom to touch the arm tefillin and kiss our hand while saying “Yotzer Or (forms light)”. We then touch our head tefillin and kiss our hand while saying “U’borei Choshech (creates darkness).” The arm tefillin represents actions we do (i.e. the mitzvos) while the head tefillin represents our mind and what we think. Therefore, some say the reason we kiss our tefillin during these times is to remind us, during the day (when it is light) is the main time to do mitzvos, while during the night (when it is dark) the main focus should be on Torah study.