One of the recurring themes in this novel is that of time. Time is something that is vitally important to the novel as indicated by the powerful symbol of Herr Thiessen's clock, which signifies the way that time has stopped still, or at least is only moving very very slowly, for the purposes of the circus and those involved in it. Note the description of the clock and what happens as it begins to change:
Meanwhile, bits of the body of the clock expand and contract, like pieces of a puzzle. As though the clock is falling apart, slowly and gracefully.
All of this takes hours.
The time taken by the clock to fully open up and the magical scene that occurs within it with the various figures performing their actions indicates the magic that is at the heart of the night circus, and at the same time the fact that the clock "takes hours" to perform its action signifies the way that the circus conflates and protracts time, slowing everything down. This is of course why those involved in the circus do not age or die, and continue in a kind of stasis, staying the same whilst they move around the world on their itinerant journey in the circus. Such magic can only occur out of reality where the normal rules of life, such as time, are suspended.
Some that our teacher gave us for our semester test over theme in this book are Family, Manipulation, Time, Emotion Vs. Intellect, Sacrifice, and Love.