You should be measuring the results of every campaign you run. It sounds like you've already done some testing. What exactly did your initial testing show? How many people did what you wanted them to do at each time of the day? Monitor this on your initial campaigns, as behaviour can differ tremendously not only at different times of the day, but on certain days of the week as well. For example, you'll almost certainly see a big difference between weekday behaviour and weekend behaviour.

To go back to your initial question though, yes, you would have thought that there's a high probability that activity will be much lower during the night as the majority of your target audience should be sleeping...hopefully