>>83391574Even when we don't realize it, our brains work pretty hard to form words, especially when you compound it with writing where you are also creating a mental image of scene complete with sound and movement. Even when you take out external factors like your health habits, stressors you've had that day, etc, if you try to push the gas when your tank is almost empty, you're gonna feel it.
For fighting it, you can improve your health habits, or time your writing at a point in the day where you feel the most energized, or you can get lucky and hit a second wind. I've had quite a few 11:46 PM spurts of writing out of nowhere after I just put down a few lines to end my day with.