I am assuming you are speaking of Elie Weisel's Night. The book gets its title from that fact that Elie, at the beginning of the book, is a true and faithful Jewish boy. He faithfully studies the Talmud then finds a teacher (Moshe the Beadle) to teach him the Cabbala against his father's wishes (because Elie, in his father's opinion, was too young). At this point in the book, Elie was full of light and God. Throughout the book, however, Elie begins to lose his faith in God and is therefore plunged into darkness. The title indicates a life away from God, but it also points to the fact that some of the most horrendous events took place at night in the book. Many of the Jewish family members disappeared in the night never to be seen again; families began being deported at night; they arrived at Auschwitz at night (and saw the horrible flames that Madame Schacter screamed about to them, causing them to beat her into silence); the moves from one camp to another happened at night; and Juliek and his violin both had the life crushed out of them at night.
I'm sure there are events in the book which happened at night that I have missed, but it is important to understand the relationship between the fear of the unknown and the night where nothing can be seen by human eyes. Fear increases at night where no one and nothing is considered safe or sacred. In the absence of God, that fear is increased by so much more intensity. Many of the Jews asked, "Where is God?" "How can this be happening to us?" So their loss of faith also meant a loss of hope. Without hope, so many just gave up and died.