קל"ה - הלכות תפילה א:ב
The Torah that one learns at night has a greater chance of rising to the Shechina, both from man's side and from the Shechinah's side. This is because at night a person is better able to nullify and attach himself to Hashem than during the day. Night is a time when the world is much more still and the concerns of this physical world are not being pursued. Also, the Shechinah comes close to the person who is learning Torah at night and listens to his voice as it is written, "Arise, cry out in the night at the beginning of the watches, pour your heart out like water before Hashem's face." (Eichah 2:19) Specifically Hashem's face, because the Shechinah is so close, and [the person learning Torah] he is literally in front of Hashem's face. At night the Shechinah dwells in the lower worlds, and the essence of her comfort is through being close to one who is learning Torah.