A lot of comments are bringing up top athletes as a counterexample, which leads me to believe they don't understand what a high-level training regimen is actually like. I have a relatively unique persepective on this[1], so I thought I'd clarify a few things. Any sport is both mentally and physically exhausting, and it only gets more strenuous as you reach the higher levels. Nobody can keep this up all day, every day…
I'm presuming the difference is that they're kids at a tennis academy. Someone is presumably paying good money for them to be there, and by the time they become professionals, they'll have spent XX years already in training, with someone else presumably working and funding their entire stay/attendance. This then presumes that those kids who do actually make it to the pro circuit aren't then the ones that also have th…
It's part of your job to be good at what you do. That does involve getting better at programming, updating your skills, keeping abreast of the newest frameworks, and learning new languages. So do it at work. If you're spending every hour of every week at work only working on the exact project in front of you, then you are doing both yourself and your employer a disservice, as you'll have lost the broad picture, and failed to solve many problems the easy way by applying a technique or tool that you could have been aware of, but weren't.
I have solved Project Euler-style questions at work and don't feel the slightest bit guilty about it. It'd be like yelling at a baseball player for practicing his throw.