Why do you think the writer titled his book Night? Why does the narrator repeatedly talk about being in a nightmare?

Since Night is a memoir of Elie Wiesel's experiences as a survivor of various Nazi death camps, it is likely that the sense of darkness evoked by the title is a reference to the bleakness of life experienced by Holocaust victims. Elie and many others faced horrific physical circumstances, including beatings, starvation, torture, and death by atrocious methods; they also endured mental and spiritual anguish as a result of enduring torture themselves, watching friends and loved ones perish, and fearing what additional atrocities were yet to happen.
Elie talks about being in a nightmare for the simple reason that his existence at the hands of the Nazis was pure agony, like a nightmare existing in reality from which there is no escape. The tragic details revealed in Night and in other Holocaust memoirs are a sobering example of the horrors men can willingly inflict upon other men when all lives are not valued equally, and when individual citizens refuse to unequivocally fight for justice in every situation.