The guzey_arena_0 test could be biased by his skill level improving over time by playing it regularly, though if he was an avid FPS gamer prior to the guzey_arena_0 tests, the asymptotic growth of skill had probably leveled off in time. I didn't see this addressed in the paper outside of this section:

"I will give myself a sleep opportunity of 8 hours (23:30-07:30). I will practice the tasks I’m going to use in order to mitigate the most extreme practice effects during the experiment. I will perform all tasks as follows:

Aimgod guzey_arena_0 at 12:30-12:50, 15:30-15:50, 18:30-18:50 and 21:30-21:50 (I also played about 2 hours of guzey_arena_0 on 2020-03-31 and 2020-04-01 while designing the experiment)"

That might not have been enough to blunt the practice effect.

Still I think it's valid data - might have been better if he played guzey_arena_0 for a period beforehand on normal sleep until his improvements leveled off.