Well you certainly seem to know what you're doing, which is good to see and means we can pretty much rule out any basic growing errors. That pretty much leaves us with two possibles, one not so bad the other rather terminal.
Firstly, your plants show clear signs of possible over or under watering. The fact that your plants up till now have been growing nicely and have suddenly wilted since the temperatures have dropped quite sharply suggests that, that could well be the cause of the wilting, despite you continuing to carefully water them, and heres how.
When the air temperature drops sharply, so does the soil temperature and if the soil temperature is too cold, it will slow metabolic rates and that means the plant will not be using very much water, as you've continued to water by your usual methods, for normal healthy growth that would have been fine, but with slowed metabolic growth due to low soil temperature it may have been too much, which is why you're getting signs of over-watering, as the roots are not uptaking so much water, they're lacking oxygen because the soil around them is saturated. This can be compounded by poor air ventilation and high humidity as the plant needs to transpire water into the air and if the air is too water saturated (high humidity) it won't be able to transpire as much as with low humidity. Humidity is temperature related and warmer air can hold more water than colder air. An 800 cubic foot grow room can hold 18oz of water at 80c, 14oz at 70c and only 7oz at 50c. So you can see how the air temperature dropping can have two affects on water uptake and transpiration - less water is used and required.
Consider installing an oil filled radiator to try and raise the lights off temperatures, make sure the plants aren't sitting on a cold concrete floor, if they are try insulating them from it by raising them up and standing them on something. Make sure theres adequate ventilation - old stale humid air extracted and fresh air brought in.
The second possiblity isn't nice and that's two plant diseases called Verticullum and Fusarum wilt which are basicly an infection of the plant water transport system by blocking off the flow of water through the xylem. If this is the case, which I hope it isn't, there's nothing you can do.
If it's neither of these - I can't help you any more! But I hope either someone else can give you the answer and your plants recover fairly soon, good luck!
