Monotonous tasks like showering, being on the toilet, etc. are definitely a huge part of it, I believe I've read that mundane tasks - your brain being freed to

Monotonous tasks like showering, being on the toilet, etc. are definitely a huge part of it, I believe I've read that mundane tasks - your brain being freed to do "nothing" while your body is on autopilot doing things it's done a thousand times before - are the most common catalyst for creativity.