The motif of night in Night is used to convey the inexpressible. Eliezer's time in the camps is something that challenges traditional language and notions of expression. In being able to use night as a motif, it brings to light the darkness and lack of illumination on moral right and wrong in the camps. The night is when the killing is at its worst. In the poem, "Never Shall I Forget," the "night," "that first night," is used to convey a period in which there is no light, no illumination.
The motif of night helps to communicate the world of the camps for Eliezer. The night motif is a symbol in which clarity about issues of right and wrong are unclear. This is used to bring out how Eliezer rejects God and how human beings dehumanize one another. The motif of night is something that shrouds human action, reflective of the reality in the camp where there is little in way of justice or moral righteousness. It is in this use of the motif of night, something recurring, in which Wiesel is able to articulate that which lies beyond the grasp of traditional linguistic constructions of truth. The use of the motif of night aids in understanding a reality in which the lack of ethical clarity and moral construction of the universe is evident.
It's fair to say that Wiesel uses the night motif in all the ways you might expect. "Night" represents darkness, a lack of clarity, and the inability to "see" things. It represents fear of the unseen and the unknown; it is the opposite of "day," when things are visible, clear cut and unambiguous. "Night" is representative of a kind of moral confusion.
In another sense, Wiesel associates the idea of "night" with completion. "Night" comes at the end of the day, and Wiesel marks his progress through the camps as a series of "last nights":
The last night in Buna. Once more, the last night. The last night at home, the last night in the ghetto, the last night in the cattle car, and, now, the last night in Buna. How much longer would our lives be lived from one "last night" to the next?
The final "night" can be understood as the death that seems inevitable, or as a kind of infinite uncertainty. Or it can be thought of as a process, or an obstacle that must be overcome—an endless succession of "nights" that must be endured. Or it could be thought of as a kind of alternate reality: the "night" of the camps as distinguished from the "day" of life before. There is also the sense that this "night" is perhaps more real, more true, than his life before the camps. Wiesel declares that "Never shall I forget that night, the first night in camp, that turned my life into one long night seven times sealed." For all the uncertainty and fear associated with the camps and the night motif, perhaps the camps teach Wiesel that uncertainty is the one thing he can count on. As one of his fellow inmates says, "I have more faith in Hitler than in anyone else. He alone has kept his promises, all his promises, to the Jewish people."