What makes you work for the whole day? (Over eight hours constantly in the deep mindset)

Working a full day or so only requires my interest. As long as it is an activity I can really seed myself in or can help me pursue my goals, I can typically sit for a few hours straight to achieve it. After a small break, I resume my work until completion. The biggest problem I have is to just sit down and do it. My motivation to give effort is lacking; however, watching video game playthroughs and rewarding myself with candy while I work has helped me get through some tough essays and excessive work.