I don't think it's about side projects. When I am on a good project the 40 hours for my employer are really interesting and i may spend more time at home reading up stuff because it makes sense. But it's hard to spend 40 hours a week with some dull, uninspiring work and then go home and be motivated do something cool. The dull work pretty much drains all energy out of you.
This has been a serious problem for me. At best it makes slogging through the workday much harder. At worst it sends me into a depressive tailspin where I feel like I am destroying my career.
