Model for diurnal nature of data
If you want to find periodic behaviors of your data, you should use the Fourier transform, which decomposes your data into a linear combination of cyclic waveforms. If you have a guess on the actual period, as in your case, you can try something simpler. For each hour during the day, compute the mean $\mu$ and variance $\sigma^2$ of your data, and plot the intervals $[\mu-\sigma,\mu+\sigma]$ for each hour. That should give you an idea regarding your hunch. (More generally, you can try a sophisticated method like ANOVA.)

Perhaps you should ask your question over in stats.se.