The definition that I mean according to the same dictionary for the word "mind" is: "the element of a person that enables them to be aware of the world and their experiences, to think, and to feel; the faculty of consciousness and thought: e.g. as the thoughts ran through his mind, he came to a conclusion people have the price they are prepared to pay settled in their minds".
Why then "Nocturnal Mind"? Personally, the necessity for me to have a pro-active diurnal mind necessitates an active nocturnal mind. When do I draw the line between night and day is the choice I make every day. I would not want the Sun and nor any Moon to have their say!
I would define my "night" as being when most people around me are asleep. This is when my mind is most active; hence "the nocturnal mind". For years, my "nights" have been long yet not at the expense of my "days" and this has been possible because the few hours of sleep that I need is hardly ever punctuated and disturbed by dreams.