Tips of leaves clawing down

In the past 24 hours the leaves have began 'clawing' down at the tips. It's been about 4 days since I last watered them and I doubt that it is from under watering. I'm assuming its a deficiency of something, possibly nitrogen. Anyone have any ideas?

could be something as simple as not warm enough, ive had that problem before with my leaves and the plants weren't warm enough, what temp do you have your room at?

Now that you mention temperature, it has changed in the past few days. I changed from 24 hours light to 18/6 two days ago. The 400W HPS that I have gives off a lot of heat and therefore it takes a lot to get cool...I think the room is getting too cold when the light is off. I think I'll just put one of the AC units on the timer with the light. Normally I keep the temperature from 72-78F.

Every problem I have had with the CLAW has been due to cold temps and over nutes.
do you have brown spots on the leaves? Is there necrosis?