I've actually slowed down a bit in recent years, however the main driver is that the work is fun. During the day I do the necessary (but not necessarily fun) work. After hours, I work on stuff that is mentally challenging and needs concentration, but only if I find it fun to work on. I do this instead of watching TV for example. As long as I get in enough time for other activities (hiking / bicycling, woodworking, designing my own creations, time with family) then hopefully I have a balance. But I have always had a slightly unhealthy fixation on sitting in front of a computer screen and creating (whether I'm doing it for work, or personal projects).