When do you measure your PH? At the beginning of the day after the night cycle? Your PH levels will be low as a result of zero photosynthesis process (leading to increase oxygen levels which helps increase PH) and an increase in carbon dioxide levels building up (which depresses PH).
If you're using a Calcium reactor, you will also see a depressed PH especially if you need to overdose (a problem in SPS tanks or heavily loaded coral tanks).
If your water quality isn't tip-top, and there is an increase in organic load (overfeeding, overstocking, poor skimming), these will acidify your water, causing your PH to drop too.
PH levels will rise during the day cycle when photosynthesis is in full bloom and will peak just before lights off. Do your PH test then.
As long as your PH doesn't drop below 7.8 and increase beyond 8.6, it's within normal operating parameters... although a stable & constant PH of around 8.2 is best but hard to achieve.
Calcium reactor users who overdrive their units can use a kalkwasser reactor to counter the low PH problem. Kalkwasser has high PH, fyi.