I sometimes dream about being & moving amongst the abstract structures that I build in my head when reasoning about a complex problem. Sometimes it gives me an insight; others, it’s meaningless, the Tetris effect in another form.

Sometimes that is really poor-quality sleep, when your brain is gnawing on a complex problem.

I've solved many difficult problems sleeping or showering.

I wake up with answers. My first waking thought is often a solution or something I failed to notice.