Think about this, do you lose a skill when tired or stressed out. Consider, for example, a welder. His work may be slower, and might not be as precise, but it would not look as if he had never welded in his life.
Even a stressed out, overtired, person experienced in process design would not suddenly forget such basics as diamonds for decisions.
Do you forget how to drive when you are tired?
No but I did fall asleep at the wheel and nearly kill myself