There are two considerations here - first the preferred body temperature (PBT) of the animal you are keeping and the second is the temp in the micro-climate (enclosure) that you keep it.
Just because the outdoor temps are hot does not necessarily mean that your hot spot in your cage will enable your pet to get its PBT. If he cannot get to his PBT then you may run into problems.
So what Saximum is getting at is if you have a thermostat then that will be controlling your heat source to maintain the correct temp for your pets PBT and you wont need to worry about turning the heat on or off.
If you dont have a thermostat then you should at least have a thermometer so you can decide whether you need the extra heat or not.
Up north I suspect that you wont need any artificial night time heating so you can turn everything off at night if you want but your day time temps will depend upon the above.
So what temps do you need to achieve for what you are keeping and what is the temp in your hot spot without any artificial heat?