I disagree. Some days you’ll be beat, sure, and you won’t be able to be creative on those days. But that’s fine, if you accept it.
Other days you’ll get a lot of creativity done in your spare hours, and if your hobby is computer science, then you can certainly get a side project done. Because if you have a few hours to watch Netflix or browse hackernews, then you have a few hours to do other things, and once you get into the habit of having fun with your hobby, it will feel nothing like work.
Granted, that last bit isn’t as easy as it sounds, and it’s probably impossible if you don’t like your side project. Like if your side project is to build x because y will be good for you, then x may not be what you want, and it’s almost impossible to do that then.
Disclaimer, I’m Scandinavian. Our work week is 37 hours. My work week isn’t though, and I still had time to build a password cracker for DES encrypted passwords in c the other day, because it was fun.
Of course, if your goal is to start a business, then it’s perfectly possible that this won’t be fun, and I do agree that it’s often a good idea to give it a shot instead of trying to do it on your spare time. But creative side projects? That’s like going for a run or watching a movie.