I am adding a bounty to this question, hoping for some scientific research results. Thank you everybody!

I have recently tried working on dark backgrounds, and it seemed (to me) to be easier on the eye. However, today I read Gerrie Schenck's comment on this answer, in which he said that mainframe developers were advised to use white backgrounds instead of black, as it is said that white is easier on the eye.

So which one is actually better for the eyes in the long run? I would be thankful for any (scientific) references about the subject, as my eyes really need some relaxation.

I wanted to make this question a community wiki, but I think that the least I can do to thank people is to reward their answers, so I'm leaving it as a normal question.

Many, many thanks for your help.

P.S. I don't know which tags would be appropriate for this question, so I'd be grateful if you could tag it in a better way than I did.