Writing fiction twists your brain in knots.
You get to experience the life of a compulsive liar. You are constantly double-checking for consistency, acting as continuity supervisor for your own imagination.
You become a detective, picking through the diary of a suspect; checking for the slip up that undermines the story and breaks the alibi.
You wake up sweating about some detail that wrecks the timeline, or breaches the established character of the person you invented.
You craft wonderful, frightening people, who you fall in love with, but must discard.
You find brilliant twists that you can’t use because they somehow subtly compromise the story.
It drains you. You can’t sleep. You are preoccupied with minutia.