My breakthrough idea come mostly when I'm exercising (read: running). It's one of the reasons I don't run with music / audio. Instead, it's just me and the road.

Initially, I thought it was a personal quirk. Then I read the book "Your Brain at Work" by David Rock. At this point, I'm due for a re-read.

https://www.amazon.com/Your-Brain-Work-Revised-Updated/dp/00...