lose one's head

Could I advise you, Commissioner, not to lose your head on this one.

Pourrais-je vous conseiller, Monsieur le Commissaire, de ne pas perdre la tête sur cette question ?

Brain fag; a numb feeling in the head as if one would lose his reason.

"Cerveau brouillé, embrumé"; sensation d’engourdissement cérébral, comme si, il allait perdre la raison.