Once the seeds are sprouted, even if it's a little cooler it doesn't affect the plants much. I started my seeds in a cabinet over one of the heat ducts where it was nice and warm. Once they sprouted I either put them in a south window or under fluorescent lights on a stand, the plants were on the floor, it's much cooler on the floor in our house. The plants did great if they got enough light. If they don't get enough light, they get tall and spindly. Of course it depends on the plants too, some do well with less light, pansies is one if I remember right!

When it started warming up outside I made a cold frame placed against the south side of our house that got full sunlight. The daytime the plastic covering the plants was rolled up so plants were exposed to the sun. On cold, rainy days and at night the plastic was rolled down over the plants.

that's how i grow my ganja!