Door Sealing

so since in using an outbuilding thats completely detached from my home and the exterior door leads directly into my room ive been having issues of keeping a regular temperature/light sealing

as is my room stays a nice ~23c/73F throughout the day no matter what temp it is outside... however at night time im having the issue of my temp dropping to 10c/45F

since the room is only 60sq ft and 100% detached from house im having a hell of a time keeping conditions constant... any one have any ideas?

is it insulated?

walls are done with r-14 exterior wall insulation
floor is done with 2" solidcore foam with plywood liner (~r-
ceiling is done with r-20 exterior insulation

get a heater

sure, know of anywhere i can get a 120vac thermostat?

http://www.amazon.com/WIN100-Heating-Cooling-Programmable-Thermostat/dp/B000E7NYY8

If you have space, have a couple of barrels of water in there. The thermal mass will raise your nighttime temps in winter and cool your daytime temps in summer. It won't be as accurate as a heater on a thermostat but it is reclaiming energy that is not being utilized.

thanks toastycookies, thats EXACTLY what i need!
rosecitypapa; theres 2 25g water bins in there already! 1 food 1 water

i got that same thermostat from ace hardware