And protects your eyesight too. I couldn't live without the 'Negative' Compiz plugin which I use as a workaround for the many applications that don't have a "Night mode".
This is an unfortunately common misconception. Sometimes yes, sometimes not. It is highly dependent on condition, in particular with what part of the field is obscured/impaired. I lack around 90% of my FOV - no central vision - resulting in ~20/200 acuity and while I find black on white to be incredibly discomforting I also don't get on with many 'high contrast' dark schemes. At the same time many of my friends with similar acuity but different conditions would be driven screaming from the room by some of my color scheme choices.
The contrast/color sweet spot varies (anecdotally, at least) greatly across both conditions and individuals.