When I was a teenager, (so long ago), every now and then I would stay up all night in my room filling a sketchbook with a bunch of doodles. At the time I was very much influenced by the Frazetta books published by Balentine. The third and Fourth volume contained sketches from the artists doodle books.
Latter on I became more interested in painting from life, thanks to the instruction of Irwin (Greeny) Greenberg at the High School of Art and Design.
Years later I started to either paint late into the night or wake up at about four in the morning to sketch. I remember hearing Greeny talk about how he would wake up early in the morning and start sketching and I had wanted to imitate that. I wish I could say it is my habit but there are times I don’t get to sketch like I want to and I wind up trying to fight my way back in to the routine.
What I would do is grab one of many reference photos I've taken (nobody is awake to pose for me) and use it to paint from. I also work from imagination either on the computer or with a black brush pen in my sketchbook.
The sketches at top and in the middle were done in watercolor in my sketchbook, the one on the bottom was done in Sketchbook Pro 6 at about three in the morning.