Because it is such light green in color. If you are overwatering(which the droopiness suggests), you could be getting lockout due to lack of oxygen. Why is it in such a small pot for that size of plant as well? Do it a favor and transplant her! Make sure it is into decent medium that can breath!
I would say the last thing to worry about right now is the night time temperatures. Closing the window at lights off wouldn't hurt though, but it should be somewhat accustomed to the temperature swings.