My answer to How does your mental performance change when you are extremely tired?
Answer by Zbiniew Malecki:
Tiredness is an indication of low energy level. Lack of energy affects whole body functionality, especially your brain.
Tired driver has slower reflex and can easily has poor judgement – >> leading to accident.
Brain need food, energy and oxygen for proper functionality. The same as tired muscles your brain can be tired too.
White-collar work is as much tiring as Blue-collar work.
Let say that you had sleepless night. Whole day will be "lost". You may have brain fog, headaches or migraine. Your ability to think fast is compromised. It will be hard to focus.
So answer is — yes it does. Mental performance will go down.