I have to be at work at 6, so I'm on the road a little after 4 (to give me some time to change and decompress when I get there), so I always ride in the dark in the morning. Most of the way is r2t, with no other traffic at all, so often I don't use lights, as there is enough ambient light (reflected city lights off the cloud cover) to see the trail. A couple of weeks ago, though, I got distracted and ran into a center post, put there at driveway crossings to keep motorized vehicles off the trail. So now I've determined to run lights whether I think I need them or not, and to change batteries (except for rechargeables) once a month, whether I think they need it or not.
And my shoulder is healing nicely, thank you.