I sincerely hope so, and I question the hygiene/health of anyone who spends 16 hours a day > 1 day a week in front of a computer. I certainly have done 16 hour stretches of coding, but I don't think anyone can sustain that for more than a day or two and still be sane. (I think you turn into rms, which can be seen as good or bad).
The message that I would like to give you is that of quality over quantity.
First off, you don't have to be at a computer to be solving your coding problems, keep a notebook, keep them in your head, write down answers as they come. Relaxing you think about a problem probably doesn't count against your concentration hours.
Secondly, if you are banging your head against a problem, it is probably time to take a break or get help with it... There are diminishing returns from struggling with a problem.
I don't doubt that you can spend more than 40 hours a week coding, but that seems like a shitty quality of life. Wouldn't you rather see the sun occasionally?
Wouldn't it be better for you to just work within the 40 hours, optimizing your use of them? Maybe find a way to get stuff done at work more efficiently, so you can spend more of your quota coding your side project?
It might also help to not try so hard when you are 'concentrating'. Let me explain:
I feel like if you are trying really hard, it takes more out of you mentally than if you are approaching it from a mentality of playing. Having more fun will definitely stretch your endurance for these things.
My mantra is to 'work' as little as possible to achieve my goals. So I like to turn 'work' things into puzzles/games. 'Work' is psychologically un-fun.