My guess is you're just seeing the normal temp fluctuation. If I'm remembering right, I thing you just have PCs for lighting? Those aren't going to heat up a tank much, if at all. I have 200W of PC
over a 46g, keep my room temp at 70 degrees, and still have the heaters come on several times a day to keep the tank temp between 78 and 80. Sounds like your tank bounces between 75 and 78, and you just happen to be seeing the high and low at the times you mention. Bet if you sat and watched it all day long, you'd see it bounce between those numbers several times during the day as the heater cycled on and off.