It usually doesnt happen over night, but what can happen is you can damage the photosynthesis process in the leaves, which will disable the leaves from turning green and producing energy. High heat will also damage the roots and make the nutes harder for them to absorb, but I wouldn't say it usually happens over night.
What color are the veins of the leaf? Are they still green? If so it could be a magnesium deficiency, if the entire leaf is yellow it is more likely a Nitrogen prob.
Can you get any pics? It you can show us some pics it would be easier to dianose the yellowing problem.
its the whole leaf thats turning yellow, and now its starting to get brown spots and the yellowing is starting on a few different leaves, i could get a picture but i dont think you could tell, i just flushed them a few days ago so the nitrogen problem is possible