I'll elaborate on what the above poster's have said, just incase your not satisfied with the answer yet.
Aslong as all your stems are not purple and the growth is not affected then the purple stems are nothing but a different aesthetic atmosphere, you don't need the night temps to drop at all unless your not running 24/0.
So if you have a hygrometer in said attic MAKE SURE the temps are not rising and dropping further than what you are thinking, also if the stems are red more than purple would suggest a deficiency in some nutrients key to healthy plants etc etc try adding some silica based nutrients into your feeding.
That will improve the strength of your plant, and the resistance to disease thus creating less ugly features during growth stages
If you have an oscillating fan on during the lights off period (if you have a lights off period) that may even be the cause due to the amount of cold air being blown all over small plant/s.
All that being said I have 5 indica plants with purple stems in flower ATM and the growth/budding isn't being affected so perhaps its just a small reaction to changes in its enviroments/nutrients etc.