Night riding on mixed surfaces has made me notice a few things about riding position.
I like to sprawl on top of my road bike, with a long cockpit and a low back. My eyeballs practically migrate up my forehead by the end of the season.
I know a more upright position works better off-road. When I ride the Cross Check on rougher trails I wish I’d set things up that way. But most of the time I’m commuting on faster terrain, in daylight.
Darkness adds a new element. For the night commutes I just put on a slightly higher-rise, shorter reach stem, to bring me back and up.
In daylight, peripheral vision around the entire eye, top and bottom as well as side, provides a lot of information you might take for granted. Night cuts that off, as vision is restricted to the patch of light thrown by the headlights. I find I want to bring the center of my visual field closer to the outer end of the light patch, especially on the unpaved bike path. That means I keep pushing myself back, trying to sit up more. Average speed is lower in the dark, so a more upright position does not have as much impact on aerodynamics.
Two-bolt bar clamps make stem changes easy enough to become a routine part of seasonal adjustment. The bike doesn’t feel that different on the road, so I may just leave it that way.