Night is used throughout the book as a metaphor for death. It's no coincidence that many of the most horrific events take place at night. The most personal of all for Eliezer is, of course, the death of his father one terrible night in January 1945. Then there's the repetition of the phrase "last night," which emphasizes death in general, not just the death of individuals:
Yet another last night. The last night at home, the last night in the ghetto, the last night in the train, the last night in Buna.
Night, night, night—all is night. Night becomes for Elie a kind of anti-dawn, always heralding a brand new chapter in his life, one considerably darker and yet more degrading. In the unremitting hell of the ghetto and of the camp, the distinction between night and day has been completely obliterated:
The days were like nights, and the nights left the dregs of their darkness in our souls.
Night is both internal and external; within the soul and without. Evil doesn't just manifest itself in individual acts, but as a corrupting, almost demonic force eating away at the souls of those attacked by its dark, unremitting power:
The night was long and never ending.
The inmates of the camp, so long as they are held prisoner there, will remain in the darkness. The absence of light means the absence of hope—and for Elie, the absence of God. Night is a permanent existential condition of those condemned to be destroyed in body and soul. The many individual acts of horror that take place at night in the story epitomize a general level of darkness and cruelty afflicting men's souls, manifesting itself in a time and a place from which the light of humanity has retreated.