This is a classic time-management issue, often titled "ants vs. elephants", e.g. using your time to tackle small tasks you can complete easily for some immediate gratification instead of investing in the large ones with big payoff. In my own experience, it almost feels like tasks have an "activation energy". I have a list of prioritized goals, but if I'm low in energy I avoid the big but important tasks and do something relatively mindless like reading Science News or doing a sudoku. In college I used to despise myself for not being able to study on Saturday. Finally I accepted it, and used Saturdays for relaxing. I know you are not suggesting I should still despise myself, or somehow trick myself into not needing down-time. But I think this "energy effect" may partially explain why we don't always choose optimal tasks.