" Programming is an intellectual exercise that requires extended periods of concentration and focus. Focus is a scarce resource, rather like manna.1 After you have expended your focus-manna, you have to recharge by doing unfocused activities for an hour or more. I don’t know what this focus-manna is, but I have a feeling that it is a physical substance (or possibly its lack) that affects alertness and attention. Whatever it may be, you can feel when it’s there, and you can feel when it’s gone. "

― Robert C. Martin , The Clean Coder: A Code of Conduct for Professional Programmers