yes they build up energy in the dark period but ive found this is only in the flowering phase during veg they dont need to do this imo.thats why ive always used 24/0 during veg as you get more growth.people who use 18/6 during veg will argue this but i have grown both ways in veg and imo the plants grow better 24/0 than 18/6 but as i say its each to their own.the postman.