Is heat causing stunting?

So flowering was started on Dec 14th, and everything has been going great. But living in my location, it gets quite cold at night. Consistently below 20 deg F (Out door temp). The room is on the corner of house, so it is directly next to the outdoors. Now when the 400w HPS goes off at night, I am concerned that it is getting too cold in the room at night. Because the light is off and I am abiding by the "total darkness", I can't exactly find out how cold it is at night in the room. Is there anyway to tell if the temperature at night has been stunting the growth here?

Camera phone pictures, sorry for the quality :-/

get a themometer that records the highest
and lowest room temps
then u'll know how cold it is
and if u need 2 add a heater @ night

A heater was already added just today. Thanks for a good idea it didnt even occur to me that they can record the temps for lowest temps.

glad i could help
i had 2 get 1 myself so i could
keep track of my night time temps
good luck