Ah, okay glad to hear that. Whenever I think of people growing plants indoors I think of dark basements and grow lights
I wouldn't think a grow light would throw off their natural cycle because you can always turn it on/off or set the hours and their dormancy is set off by the cold. I think semps are probably like tarragon, without a good chill they will just get weaker and finally die.
Speaking of cold, interesting to think about getting them some chilly weather in an apartment. Aside from cracking the window (brrrr!) you could try moving them closer to the window to get some cold air seeping in. Unless your flat has really good insulation
In that case you could always try putting them in the refrigerator at night too I suppose.