I wouldn't say white on black, more like grey and some other muted colors on black.

I remember making the switch from black on white. I was working a contract job with ungodly hours and my eyes were pretty much always red and irritated. I started to notice certain colors on the screen were particularly irritating so I started to fiddle with the color scheme and eventually ended up with the light on black scheme.

It was astonishing how much it helped my eyes.