My personal anecdote:
I find that I tend to solve "hard" problems during periods of unconscious processing (times where I'm NOT actively thinking about the problem). However, I also know I need to spend a lot of active time thinking about the problem first, before my brain can "take over". I'm usually able to structure in the unconscious processing time with things like exercise, in particular long distance hiking (5+ hours ideally) on weekends.
Finding time to reflect on problems in the middle of the week, in an active sense, has been my hardest challenge. As someone who codes all day for my startup, I can easily get into a groove for 10+ hours and not take any meaningful reflection breaks.