And what about when you have two jobs - 40 and 20, when they are unrelated lets say - front end development to pay the bills and number crunching side project f

And what about when you have two jobs - 40 and 20, when they are unrelated lets say - front end development to pay the bills and number crunching side project for fun - they exhaust differently. There are a lot of developers that put more than 20 hours weekly on side projects or professional level hobbies. It is probably the more of the same that makes the quick decline after 40 hours.