At dawn and dusk, aka twilight, the sun is below the horizon. So you only get indirect illumination. Contrast is terrible, and shadows will be diffuse. The dire

At dawn and dusk, aka twilight, the sun is below the horizon. So you only get indirect illumination. Contrast is terrible, and shadows will be diffuse. The direction should still be mostly right, but you will be hard pressed to define the end of the shadow. The position of the sun can be a bit weird depending on the season the further away from the equator you are. Especially in the periods where the sun never rises above the horizon. Other than that I don't know exactly what problems you might have. The midnight sun perhaps flips the direction?