The pH change is not because of your tank's temperature. They both happen at the same time, but that doesn't mean one caused the other.
The reason your temperature swings is because the temperature is warmer in general during the day.
The reason pH swings is because CO2 in the water decreases. The reason that CO2 in the water decreases is because plants use it when photosynthesizing. The reason that the CO2 decreases only during the day is because plants can only photosynthesize when the lights are on, which is almost always during daytime hours.