68 is ideal. If I am awake, I can cope up to 71, as long as the air isn't still. It has to be moving. I have several fans. My body could stand warmer, but I can't breathe if my head is hot.
At night, I'm actually comfortable down as low as the low sixties. I just burrow down in a couple of quilts, pull them up to my neck, leaving only my head (and occasionally one foot) exposed. I can't sleep in a hot room, wake up with allergies flaring and a bad headache.
I have radiator heat, which I love because the heat is low to the ground. If it gets too hot, since I can't set a precise temperature, I just crack the windows even if it is near freezing outside. Friends have joked that they can see their breath in my home, but I've always been that way. My mother is the same.