While Elie Wiesel likely never elaborated explicitly on his choice of the title for "Night," it's responsible to conclude that he selected it for its extraordinary symbolic power. The theme of entering darkness operates metaphorically on many levels throughout the book in Wiesel's physical and psychological experience of the Holocaust.
In an initial respect, the metaphor of night marks the end of young Elie's normal life, when he and his family are violently uprooted from their homes and community and sent to the concentration camp at Birkenau. During his first weeks there, Wiesel experiences the total loss of all his female family members. He also witnesses, along with his father, the fiery death of a truckload of children. In essence, night represents the descending darkness of a complete absence of humanity and compassion, as well as of the lunatic, comprehension-defying, physical articulation of Hitler's plan for a world without Jews.
In Wiesel's interior aspect, night represents the increasing loss of hope, the numbing of his senses, the anger and alienation he feels towards and from God. As Wiesel continues to lose his former idealism and the solace of prayer, he nonetheless continues to seek out God somewhere in his experience, despite every effort of circumstances to erase him. Despite his continued inner search for faith, Wiesel must still depersonalize the death, misery and catastrophe all around him. He must turn his mind to scavenging and obtaining food and to other elements concerned with survival.