I'm working on a Master's Degree, and the level of work requires some long, long reading and writing sessions. Just this week I wrote a blog post on how I can use Angry Birds to explain the various theories of learning - you can catch it at http://highlycaffeinatedyarnbender.wordpress.com/ .

For a self-check on the tiredness-to-productivity quotient, I usually play Free Cell Solitaire. It's a game I normally run 40-50% wins, sometimes more. If I play 4 games in a row without a win, I know I'm too tired to properly evaluate and apply what I'm learning, so it's time to quit!