I work 7am-4pm (which means 5am-6pm if you count commute time), and though there is plenty of time to work on my own projects in the evenings, I find myself mentally drained most days, so what takes me an hour to do at work takes 2 or 3 at home in the evening. Weekends are more productive.

And starting at 7am. That's nine hours out of the day. Even if you're not 100% busy during that stretch, you're still not free to let your mind wander and do the things that matter to you. You're mentally occupied.

That's busy enough. See comments about working on someone else's schedule, first, then add in time for meals, commuting, showering, taking a dump, exercising, doing household chores, paying your bills, responding to email, etc.. After all that, there isn't much time left in the day. Especially since some businesses start closing down at night. You're probably tired, anyway. Those 9 hours at work make a HUGE difference.