What happens during the dark cycle?

I'm new to this whole growing thing. I'm currently into my fifth week of flowering on my third grow and first using hid lighting.

I am not new to reading which I do in abundance. With that said, I am not clear as to exactly what is happening with the plants during the dark cycle? Is the plant doing its growing during this time using the energy it made during the light cycle? I've read thc is formed in the dark cycle and the plants renew their thc making every night, whenever that happens to be in your grow area.

Anyone know what's going on in the dark?

well if ya like to read check this out

http://academic.kellogg.edu/herbrandsonc/bio111/metabolism.htm

not about MJ but will give ya an idea