That is absolutely the case, but because most screens emit light and white is the brightest colour possible it also makes the eyes more tired (in my experience,

That is absolutely the case, but because most screens emit light and white is the brightest colour possible it also makes the eyes more tired (in my experience, not sure if this is actually the case). So you have to find a balance between readability and comfort. I personally think that #000 on #FFF is too harsh (for the purposes of reading large texts) and think that slightly less than maximum contrast would be preferable in most circumstances.