[flagnog]

way back in the dark ages, before the Playstation and XBox and Nintendo, I conducted my own empirical studies using coin-operated arcade games. I found that after a couple of hours, my reflexes plateaued, then degraded. I had to go do something else for a couple of hours before I could come back and do better.

[rmk2]

Another really easy way: I have a simply memory game on my phone, 6x4 fields. If it is too late or if I am tired, I will make more mistakes and take longer to memorise things. On the other hand, I can turn and solve it much more quickly when I am fresh and well-rested. Also, if I play a number of them at a time, they start to run together, again resulting in worse results.