The close to the clubhouse ranges at my club face east. This time of year, late autumn/fall, the sun tracks well to the north, on the left hand side of the ranges. This creates the effect that when the sights are alighed properly there is more sunlight down the left hand side. Instinct is to move the front sight to the left to get the light equal down each side, moving the point of impact left.
A few years ago, about this time of year, I was running a match. One of the shooters suddenly found his group had moved to the left significantly. I told him about the effect the sun would have at the time of morning and he didn't believe me. I told him to stand in the shadow on the left hand side of the range, and his point of impact immediately moved back to the target centre.
Yesterday I was sighting in a new load in one of my .45ACP 1911's. It was mid morning and my first group went well to the left of centre. It came back again when I concentrated on the sight, not the light.
I guess holding a torch in your left hand close to the gun at night would have the same effect.
For several years now we have had a WML attached to our Glock 17 duty pistols. There have been stories of cops whose flashlights have given up functioning while they were armed, drawing their Glocks to use the WML. Not recommended and actively discouraged by the brass for obvious reasons.