Honestly, it happens when I get some weird idea in my head that I can't stop thinking about, and a sort of mania takes over and gives me the energy and drive to work on it for hours on end.
Ditto, that kind of mania is a force to be reckoned with. On one hand you get insane productivity and momentum from it and on the other hand can cause you to momentarily neglect some responsibilities. I have a love/hate relationship with that side of mine.
That's about the only time I can do honest work programming. It makes me avoid doing programming tasks at work and try to find more writing-related stuff ...
It helps when the end goal is really well defined like a software emulator. Nearly everything that could slow you down is a known quantity. Conversely, the unknowns, like how to implement native hardware API X on non-native non-accelerated platform Y, are naturally where community emulators are weakest.
(not that you should take advice from random strangers on the internet, but) that sounds like classic ADHD hyperfocus to me. Giving a name to it might help in figuring out how to maximize its benefit.
I experience the same hyperfocus, but I don't have ADHD, I'm quite the opposite really.
My background is design and when I was a child I could easily 'get lost' in various forms of art etc for hours. I've met plenty people over the years that are the same, so I'd argue it might be something more aligned with some creative and analytical thought process, and a drive etc to understand/solve the problem at hand.
In terms of where to find the time, there are 24 hours in the day, at least 8 for working the 9-5.