I will try to answer your question for you the best I can.
During the light cycle the plants absorb the light & nutrients; convert it into stored energy and store it in the large fan or sun leaf. The plants do grow in the light cycle and they also grow in the dark period but they also do different things in each cycle. The dark cycle is the rest time the plant needs to use up the stored energy that it produced during the light cycle. It is this time that the plant moves around needed minerals, nutrients and sugars to other areas of the plant that may be lacking. There is a whole transportation system with in the main stock of the plant. This system works best in the dark cycle to move around the food & energy it needs to for make for a happy healthy plant.
That is the short version I hope if answered some of your questions.