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I suggest 10-30 min naps when you really need a break. Also, as you suggest, short bursts followed by breaks is a great approach. Good luck on 100 kanji a day. Keep us updated on your progress!

For some reason, I find that snacking extends my effective concentration time considerably. Almost anything will do - peanuts, raisins, crackers - although gum doesn't work as well as real food. There's something about having my mouth working on something that keeps me more alert.

For my breaks I like playing pool - lets me shut down my brain for a short while, and chatting with people while playing seems to bring in some new ideas for stories.