The Land of Nod is where we go when we are asleep. It is a place which cannot be recalled once we are awake.
The term 'the land of nod' is used as an idiom and the idiom 'in the land if nod' is used to convey the phenomenon of being asleep. But this use of the idiom is used mainly in old form of English and sometimes you can use the idiom in a humorous way for somebody who is sleeping. The sentence that can help you in understanding the term is given below: John and Jack were in the land of nod and meanwhile I took out their calculator.