Night does these things to you. It makes you paralyzed. Most angst-provoking of all to the young Wiesel was his loss of faith in God, and this is the brunt of his book and the brunt of his theme throughout his life, no doubt intensified by his later philosophical studies under existentialist teachers such as Buber and Sartre.

God was killed but, in another inversion (day into night), God was killed by those He created. He, the alleged potent Being, had been made impotent by so-called impotent beings and was dying on the gallows along with a child so light in weight, that when hung, the boy died slowly and in agony:

I was the accuser, God the accused. My eyes had opened and I was alone, terribly alone in a world without God, without man (Night, p. 64.)

Night is the umpteeth level of alone-ness. In the day, a friend can hug you, reach out to you, whilst another can physically touch you. In the night there is only, and you alone:

"Here there are no fathers, no brothers, no friends," a Kapo tells him. "Everyone lives and dies for himself alone" (p.23)

Day had gone. The optimistic naive dreams of the sheltered boy who had dreamt of a messiah was replaced by an unimaginable nightmare - by a long night; and the worst of it seemed to be that his constant succor and hope of the past -- 'the Rock of the Ages' had flitted away with the day and vanished in the smoke of the crematoriums.

Wiesel's experiences changed him as they changed others in differential ways. As regards Wiesel, they transformed him from a naive protected youth into a cynical resilient man.

Important is it to note that the book itself -- true to its title -- is no such clear formulation of unvarnished day either. Originally written in Yiddish, it was translated into French and, to please an audience, stripped away to the extent that...

The original Yiddish was angry and savage, and elaborated far more. The French version, and, later, the American version, subdued that anger and directed itself to its peculiar audience (Seidman, 1996). The book, as a result, has been pronounced as being a new state of autobiography, possibly a fictionalized autobiographical memoir (ibid.). In its way, it epitomizes, in one more aspect, its title 'Night' where reality masquerades under a guise that appears real but is, actually, the obverse.
Night need not be totally pessimistic. Wiesel's 'Night' was the first book of a trilogy -- Night, Dawn, Day, and, as Wiesel himself observed:

In Night I wanted to show the end, the finality of the event. Everything came to an end -- man, history, literature, religion, God. There was nothing left. And yet we begin again with night (Wiesel in Reichek 1976, p. 46.)

Night is an inversion of day; of order into chaos, of powerful into powerless, of sane into insane, and throughout it all people react in varying ways. Wiesel writes from his stance of anger, demoralization, and bitterness, and focuses on those who betrayed others, or acted vile, but there were others, too, who portrayed heroism even at the most trying moments comforting each other to the last. The Holocaust has many stories of heroism that Wiesel omits. Written from his existential perspective, Night may be as much philosophical treatise as it is memoir. By covering one theme, it omits many others. and, finally, night can be seen as pure blackness, or as the start of a new day.

References

Reichek, M. "Elie Wiesel: Out of the Night," Present Tense. Spring, 1976, pp.41-47.

Seidman, N. "Elie Wiesel and the Scandal of Jewish Rage," Jewish Social Studies, December, 1996

Wiesel, E. Night. USA: Bantam Books edition, 1982,