Why does it say specifically night?
Rashi: "Laylah" is a time of affliction and darkness.
Radak: When I waken from sleep in the night, I remember Your name and unity, and think about it.
Malbim: This is like "Shachavti va'Ishanah Hekitzosi Ki Hashem Yismecheni" (3:6). He was in great danger that night due to Achitofel's counsel (to pursue and kill him), just he asked Hashem "Sakel Na Es Atzas Achitofel" (Shmuel II, 15:31). Hashem (Midas ha'Rachamim) saved him.
Why does it say "v'Eshmerah Torasecha"?
Radak: I know that guarding from Aveirah is a great ladder to ascend to knowing Your name.
Malbim: I know that I was saved in the merit of Torah. Also at the time that one does not engage in it, it protects and saves.
