An experiment I have done has shown quite large fluctuations in concentration of H2O2 when left to decompose in a variety of conditions. The concentration rises at night and falls during the day. Could it be due to the heat decomposing it then it reforming at night?

If you are leaving the stuff in an open container, all sorts of material will fall into it. This would catalyze the decomposition quite nicely. But having the concentration rise is another matter.

So my presumption is that you are studying decomposition in a closed container. My guess is that the container contains organic material (finger grease, for example), which is oxidized reversibly by the peroxide. The industrial synthesis of hydrogen peroxide uses a process in which water is oxidized by an organic oxidizing agent; perhaps that is what is happening here. During the day, heat or light might drive the reaction in one direction, while during the night it reverses itself.