I do both. I'm currently developing my own startup thing on the side, working full time as a web developer, and doing web development/design work for clients
I definitely find this to be true, I think it's more to do with the fact that you're working for yourself and setting your own guidelines rather then having set hours or a strict set of tasks to do at a regular job.
Do you find this to be true? If yes, do you think this is because of the clients that you work with?