Japanese to English Medical Medical (general) protocol

It is when we are very tired because of lack of sleep or something, we can't think well or study effectively.
Is this condition called "brain does not work"?
This is very simple matter however I realised I am not really sure.
I also felt like by the phrase "brain does not work", brain function itself is not working properly, like causing paralysis or sth like that.
It is not like medical urgency but simply in a daily life, by such as luck of sleep, when we can't think well... how should I express 頭が回らない in English?

Thank you so much in advance.