With the proviso that n-back performance often improves over (at least) the first few weeks of daily training. So if Mark_Gomer & his friend start n-backing at about the same time they change sleep schedule, they might get a spurious improvement in n-back scores.

Good call; we'd figured that we would just notice if our cognitive performance declined, since we perform lots of cognitive tasks daily, but maybe we should be suspect of our ability to judge our performance. Does anyone have any suggestions for test we could use? Would, for example, my mean Tetris score (or distribution of Tetris scores, since I could get better or worse with each passing game as my brain got more used to Tetris or as it got more tired) per waking period (not that I plan to play Tetris every waking period), be a good reference? EDIT: Tetris sounds like a really stupid way to measure cognitive skills, now that I re-read this. I guess it was just the first thing I thought of that I could easily keep quantitative track of. EDIT 2: I play a lot of Tetris (~1 hour a day, but not usually for more than ~20 minutes at a time), and it is the only video game I play.