I had read somewhere that as we age we need less of most things, food, sleep etc. And when we're really old; air. I think you get a massive bit in middle age where you don't need as much sleep: you stop growing, stop learning in the same way, day to day life and job becomes routine and less of a challenge, so you need less recovery time. When I got my new job, I was exhausted all the time, I was less active as I went from being on my feet all day, to being behind a computer, but learning new skills took it out of me. I then recovered and went back to normal sleep patterns. Broke up with wife, tired all the time.
I guess when you're happy and content, your body and psyche takes less of a battering, so needs less to recover. Maybe you're really content with life and in a good place, having a gf and reading sounds ace to me.