My ORP readings do show some increase at night but it's not very much.
Here is a 48 hour snapshot of my ORP readings. You can see an average of about 40-50 or so difference between night and day. Those occasional peaks that you see correlate to pump starts when fresh pool water enters the sensor bypass manifold.
I should note that I am NOT relying on ORP readings for the determination of the sanitation effectiveness of my pool at this time, due to my current CYA levels of 70ppm. Just observing and learning at this point.