Serious question. Which is easier on the eyes (less straining for long periods of time).

Black text on white background. OR
White text on black background?

People usually say black on white, but I find it quite stressing and do the opposite. Maybe it depends on the person.
@drymer I think white on black tends to be better in low light but it normal lighting conditions I don't know if there is a favourable one over the other.