Heat does not linger for hours, if it's being vented-out. Is there a way to open both, the growroom door and the tent flaps at night without exposing to light leaks? You'll want to come up with an arrangement that allows the fresh air overnight every night.

Either you aren't venting enough, or you have a heat source providing extra heat.

Are you fan-forcing the heat out, or just using the 'heat rises' technique?
Fan forcing the heat out makes a negative air pressure in the growroom, which forces fresh (cooler) air into the growspace.
If you are only fan-forcing the fresh air in, you lose that control over the heat, and overclock the ability of the vent to remove the heat. (mixed air gets vented out, rather than the heat) But regardless, the heat shouldn't remain for more than an hour after lights-off.

Is your growroom above the oven...?
Is your growtent in the water heater closet...? (had to ask...ya never know. :jointsmile: )