Why do the alarms only come on at night in Farenheit 451?
In the society of Farenheit 451, much as in Vonnegut's short story, "Harrison Bergeron," there is a forced equaiity;
We must all be alike. Not everyone born free and equal as the Constitution says, but everyone MADE free. Each man the image of every other; then all are happy...
No free thought, such as is found in works of literature can be allowed; therefore, books are burned. They are burned at night because against the background of a blackened sky, this burning is dramatic. In Part I of the novel, Bradbury writes,
Always at night the alarm comes. Never by day! Is it because the fire is prettier by night? More spectacle, a better show?
So, even at night when one is alone with one's thoughts, there is the warning that such thoughts, be they literary or internal, will be eliminated in the goal of "equality" of thought. The night fires disturb these inner thoughts.
In addition to the spectacular effect of night fires and destruction of thought, there is also the suggestion of the mystical which later appears at the novel's end. As the book people advance up the river at the end of the narrative, Montag ponders that the fire represents the end of the old society, with the birth a new hope in the Phoenix that rises from the ashes of the burning books:
A time to break down, and a time to build up. Yes. A time to keep silence, and a time to speak.
There must be free thought, else man cannot live. Montag now understands why people have tried to kill themselves as his wife has done. Without the thoughts of others by which to measure oneself and to grow, people cannot be fully human, and they despair. They must be reborn in thought, or they cannot be happy.