Being physically tired/fatigued is only one kind of stress.
Brother, that is the truth. One thing our pistol combat instructor hammered into our heads is that in the stress of a split-second, face to face shootout - by that, I mean when a bad guy is SUDDENLY pointing his cocked gun at you and you have a fraction of a second to either put him down or die - all kinds of things will happen:
- you will get tunnel vision
- your active thinking will basically shut down; you will be unable to mentally process what you see and hear
- you will stop breathing
- your body will stiffen
- and most important: whatever action you take will depend on your training. Whatever you've taught yourself to do instantly, automatically, reflexively, without thinking, if anything... is what you'll do during that fraction of a second. Or you will die.
This physical and mental state can NOT be simulated by playing paintball, shooting in competition, running 400 meters, blasting holes in a paper target, watching Rambo movies, or by reading posts on internet forums.