Why did I develop an interest in this? I did not play guitar for an extended period of time. I didn't have time or energy. Life got in the way. But, one thing that I never gave up doing was playing guitar in my mind - pretty much all the time! Then whenever I picked up my instrument just to noodle on it bit to relax I was quite surprised to see that I was still able to play - some pieces even better than when I actually practised them. I didn't know why this was at the time. Just assumed that I was lucky or that I practised really well when I learned them. Now I think it was due to my constant mental practise. In this context, I highly recommend this book titled The Brain That Changes Itself by Norman Doidge. Excellent treatise about neuroplasticity - how you can change your physical brain simply by thinking about it.
These days probably 60% of my practice takes place completely in my head. Meaning, once I've deciphered the piece I'm working on, I set aside time to practise it in my imagination, visualizing my hands, fingers while hearing the music in my mind. Make no mistake, mental practice is not easy. It is not just playing the melody haphazardly in your head. You need to make an effort to imagine yourself on the instrument performing the piece feeling every single note, every single position - left hand, right hand alike. You have to go over difficult passages repeatedly, play some parts slowly just like physical practice. It's surprisingly effective! Also, you don't have to worry about straining your fingers, disturbing the neighbours, wearing out the strings, and you can pick the fanciest, the most expensive guitar you wish to play on. Give it a try...