Idiots at work here. Subjective questionairres are always a total waste of time. They should have given participants a performance test of some kind. I will wager the gamers would perform as good or better than the non-gamers.
They did take objective measures, via polysomnography. It's one of the novel aspects of this study. Interesting that perceived difficulty getting to sleep after gaming was greater than that recorded via the polysomnography.