You have to either run climate control all the time based on timers that turn on extra exhaust during lights-on and run your lights at night to even out the heat, or put your systems on a thermostat- a good one, that turns on a/c above 75 degrees and turns on a space heater below 65.
Attics are doable but they kinda suck, you really need to treat your attic as if it were a living space if you want to have a successful grow up there. I grow in the attic, it's a walk-up with a couple windows, I have to run a/c from April to September and a heater from Thanksgiving until April Fools.