In Ray Bradbury's Fahrenheit 451, there is definitely a great deal of evidence that the firemen work predominantly at night, mostly in the wee hours of the morning. In one sense, setting the firemen's work at night corresponds with our actual real world. In another sense, setting the firemen's work at night helps to symbolize both the clandestine activity of the citizens the firemen visit and the dark, evil, manipulative nature of the firemen's work.
One piece of evidence that confirms for us the firemen primarily work at night can be found within the first paragraph of the book. The book opens with a vivid description of Guy Montag taking great pride in burning a new set of books. Most importantly, within this opening paragraph, there are also a couple of sentences containing evening imagery. For example, the narrator describes, "He flicked the igniter and the house jumped up in a gorging fire that burned the evening sky red and yellow and black." Next, the narrator describes, "He strode in a swarm of fireflies." The images of "evening sky" and "fireflies" tells us this moment is most likely taking place around the typical dinner and post-dinner hours, between 6 pm to 9 pm, which are actually the peak hours for fires in the real world. Later, the narrator describes Guy walking into the firehouse at 1 o'clock in the morning, which we know from the narrator's description of what the "time-voice in the firehouse ceiling" is saying: "Thursday morning, November 4th, ... one thirty-six ... one thirty-seven a.m."
In the real world, while fires can happen at any time, fires most commonly occur during the typical dinner hours because many fires are a result of cooking accidents. It's also very common for nighttime fires to occur in the wee hours of the morning due to human negligence with cigarettes and electrical equipment (Ready, "Home Fires: Prevent Home Fires"). Hence, in describing the firemen working at night, author Bradbury is reflecting what we already know to be true in the world.
However, it can also be said there is symbolic significance to setting the firemen's work at night. Night is a time of darkness, and darkness can often symbolize evil. Night is also the time of day in which most people are at home, so night is the most frequent time that neighbors would spy on neighbors doing illegal book-reading and the most likely time that such illegal activity would be reported. For example, due to the earlier time report from the time-voice, we know it is between 1 and 2 o'clock in the morning when Mrs. Blake reports the location of books in the old woman's attic, the old woman who Guy witnesses being abused by Captain Beatty and also witnesses burning herself up along with her books using a kitchen match. Hence, the author uses the darkness of the night to symbolize the evil of a society demanding its citizens spy on each other. The darkness of the night further symbolizes the evil of a society forbidding its citizens from gaining knowledge and their own ability to think through books.