"Night" is not only the title, but a recurring motif in this story. Its significance is both religious and emotional.
Probably the best and most repeated example from the story comes in the third chapter, after Elie and his family arrive at the first concentration camp, and he is separated from his mother and sisters:
Never shall I forget that night, the first night in camp, which has turned my life into one long night...Never shall I forget those flames which consumed my faith forever... Never shall I forget those moments which murdered my God and my soul and turned my dreams to dust. Never shall I forget these things, even if I am condemned to live as long as God Himself. Never. (32)
As a religious symbol, night represents the absence of God. The narrator expresses that with each unfolding horror, a little more of his faith is killed. It is important to note however that he "did not deny God's existence, but I doubted His absolute justice" (42). It isn't that the experience causes the narrator to believe God never existed. Instead, in the presence of raw evil, he is convinced that God cannot be present.
In addition to this religious explanation, the night motif is further carried out as a recurring parallel of the existence of darkness and evil, and the association of night with death:
"Yet another last night. The last night at home, the last night in the ghetto, the last night in the train, and, now, the last night in Buna. How much longer were our lives to be dragged out from one 'last night' to another?" (79)
Each "last night" brings more horror and more death. Every night comes with the fear of not waking up the next morning or not living through the next day. It is as if the narrator and others are simply waiting to die.