>> if i constantly always have my mind on it. i will always want to be doing that. a glor: sully zack says he's not an addict, but he can't remember how young he was when he started playing video games. >> i was probably four or five years old when i was old enough co actually get my hands around the controller. ow reporter: now 19, he spends hours a day on gaming systems. >> if i'm hanging out with a e iend who also really loves games, we might just have a marathon and play for like eight hours. u reporter: do you have to be conscious about taking a break or getting up from the video game? >> it is difficult sometimes, yeah. >> reporter: from difficult to compulsion, that fine line has noe world health organization now defining gaming addiction. saying, "if people have impaired control over gaming, give it priority over other activities, and there is a continuation or escalation of gaming despite thgative consequences, they iould have a mental health condition."