From the Washington Post today:
Having to make too many choices can affect one's ability to stay focused, finish work and do complex mental tasks, finds a study in the Journal of Personality and Social Psychology.
Almost 400 people took part in seven experiments in which some were asked to make choices or rate various products. The more choices individuals had to make and the more time they spent deciding, the worse they fared on later tasks, regardless of the complexity of the choices.
That's what makes writing so difficult. Every word you write, every turn of phrase, every decision to add or omit content -- they all involve decisions. Deciding whether to expand on a point or whether you've done enough explaining can leave you feeling as though you've run a marathon.
This makes it clear why binge writing is so bad for you. It literally drains you of brain power, so that the next day, and the day after that, you feel ill when you consider sitting down to write.
Just writing this has exhausted me. I think I'll go lie down (see my post on sleeping).