I searched/found this thread after noticing that they seem to grow faster after having a bit of darkness. Not just taller, but leaves getting bigger as well.
Here's what I found after reading this thread:
http://en.wikipedia.org/wiki/Calvin_cycle
This is my interpretation:
Basically when the plant is exposed to light, it is in daytime mode. Photosynthesis allows it to absorb the sun's energy and store it in sugar. It may grow also, but the plant is primarily in what I'll call "energy absorption/storage mode."
When in darkness, the plant "knows" that there is no sunlight energy and is in nighttime mode. This is when it uses the energy that's been stored to make itself stronger and bigger in order to absorb and store even more energy when it does see the sun again.
It's possible that plants that produce flowers/fruit and die may create and store more sugar, and grow more at night, because it is going to die when the light becomes scarce and it needs to have stored enough to reproduce. A bush that doesn't die would have no need to store as much energy or to grow as big as possible since it doesn't have as much need to reproduce.
This may be completely false conjecture, but evolution tends to lean towards logic and survival.
You're plants may not be happy with 24/0 because it is constantly synthesizing and it would need more than the recommended nutrients for a natural grow in order to store energy. It will become deprived of at least a few nutrients or two.
My plants are in Root Organics coco, with some higher end nutes, using a hybrid setup of sunlight as well as 200watts of flouro. They're growing ridiculously fast and it is very noticeable after they've seen darkness. I've been inadvertently alternating on 24/0 and something like 18/6 so basically 42/6. During the 24/0 days it almost seems halted/stunted in terms of growth, and after the 4-6 hours of darkness it's like an explosion.
I'll try 20/4 the next few days and see what happens.