Night is "cover of darkness" also.  We must not forget to consider color symbols...black is almost never used in a story to indicate positive outcomes or personality traits.  It is in the dark that people who intend to do evil deeds play them out...so they will not be seen or caught.  In this book, the night symbolizes cold, lonely, and unfriendly times.  People freeze and starve and die in the night. 

Night is a concept that changes for Elie and the Jews of Sighet throughout the novel. At the beginning of the novel, night is a place where Elie feels that he is safe, in fact it is the only place that he finds solace because during the day the Jews of Sighet were being "relocated" to work camps, but at night, no one left. By the time Elie arrives in Auschwitz his view of night has completely transformed. He sees the flames and faces his own death, he even considers taking his own life to avoid burning alive. By that first night Elie recalls that, "Never shall I forget that night, the first night in camp, that turned my life into one long night seven time sealed." (Chapter 3) From this point on night is no longer safe, it is cold, it is hungry, and it breaks the spirits of even the most spiritual men.

See the link below on themes to read more about the role that night played in the novel.

It is very significant.  The title of the book refers to the fear and safety that the cover of night brings. Night is the time when the SS came for the prominent Jews in the communities both before the Jews were evacuated to the concentration camps and also in the camps themselves.  In this regard, night was a time to be feared since you couldn't see what was coming.  In the book, the victims of the concentration camps could let down their guard a little at night where no one could see them, and they could cry or not be so careful with facial expressions.  In this regard, night was a time of comfort and relaxation.  It was a cruel, double-edged sword.

One important quote in the book which really points to the purpose of the title is:

Never shall I forget that night, that first night in camp, which has turned my life into one long night, seven times cursed and seven times sealed...Never shall I forget those moments which murdered my God and my soul and turned my dreams to dust.  Never shall I forget these things, even if I am condemned to live as long as God Himself. Never.     Chapter 3, page 32.