pH is related to how much CO2 is within the system. During the day, photosynthesis is taking place and CO2 is being "fixed" by plant activity, thereby reducing free CO2 which when combined with water is acidic(lower pH). Less free CO2 causes the pH to rise. At night plant respiration, where they actually give off CO2, shifts the free CO2 in the oppositie direction and you have a net buildup which again lowers the pH reacing a peak, or in this case lowest pH level in the early morning hours.
The absolute pH number at a given time is not as important as the stability. Quick changes in pH are bad, but slow changes over the course of hours are generally nothing to worry about. As the post above notes, keep your carbonate alkalinity above 80 ppm and you should be fine.