A cot without good insulation can be brutal. I have an insulated air mattress and put a piece of ensolite on top of that. I've slept warm down to 5 with it so far.
A couple of years ago, I backpacked into timberline at 11,000' elev. It was mid-May and a cold front was moving in. I had a down sleeping bag rated at 5°, a Thermarest air mattress and a foam pad that was only 4' long. When I laid down to sleep I realized that there was a leak in the Thermarest. That short foam pad saved me from getting up in the middle of the night and packing out. Only my feet got cold, but I laid my backpack at the foot of my sleeping bag and used that to keep my feet warm.
