I'm old enough and I have done this exact thing enough to know that of course it suffers. A lot more than you want to admit too.
Even if you don't ever mix the 9-5 time with the freelance time, (Which FYI is nearly impossible in practice.) you just aren't going to be at your best when you show up the next day to work your 9-5 8-hour day after having worked a full day and moonlighted for another 4-6 hours afterward the day before. You're lying to yourself if that's what you think. And if you're working weekends, you're going to get burned out in a way that only punishes your 9-5 employer for not giving you a higher workload. People get fired all the time when their weekend or after work binge drinking interferes with their work the next day, why should spending your discretionary time burning yourself out be any different?
Human beings can only do one thing in any moment. Worse, this truth has some applicability over longer time scales as well.
Human beings require substantial periods of recovery in order to perform optimally. Often, what you do during that recovery has dramatic implications on how well you're going to do when you get back at it. (Ever solve a difficult programming problem only after taking a break? You won't get those same eureka moments if on your breaks your loading entirely new problems into your brain all the time.)
I work design/construction/commissioning engineering industrial machinery and plant, including automation/controls software.
On site standard days are 12 hours, crunch time 14's become normal. Depending on project this might be month on month off, 5 days on 2 days off, 8 + 6 days, sometimes every day 3 months straight, or more.
You do get used to it and can be overall productive for the hours - usually company is paying (big) by the hour so they wouldn't be paying if they are not getting.
Not everyone can do it, or wants to, but different people have different capabilities.
because by definition you're being paid for your time. I've never seen a non-contract tech position scoped as "here's an exhaustive set of responsibilities; you'll be paid 40 hrs/week to accomplish them but if you finish early the time is yours". According to your profile you're a new EM; can you honestly say there's a point in your day when the job is done-done and you can jump over to your other job?
Depends on the manager and the type of work. When younger I was working as a part of junior admin team for big ISP. Every day we'd all get the roughly same amount of maintenance tasks to do - but since, unlike other guys, I was into programming I automated a lot of it (and probably was a little faster and more focused in general) - so I'd finish it in about a half of the time they needed. Now I was quite clear that I will not do more work than others unless I get paid more, and the manager wanted to keep me, but because of bureaucracy and syndicate I couldn't get the raise. So we stroke an internal deal that once I clear my tasks I'm free to do whatever I want with my time, as long as others don't know about it.
Now, I wish I could say I did something smart with that time, but it was a long time ago, so I mostly spent that time playing Starcraft or chatting on IRC :)
