There are several issues with the translation you're suggesting there.
Let's start with the English sentence:
I am going to sleep tomorrow.
The way you've parsed it to translate "going to" is taken to mean the motion verb "to go". But is sleep a place that you are going to?
Unless, this is some really poetic English, I think less colloquially what you are saying is either:
I will sleep tomorrow [interpreting go as the future tense]
in which case the translation I would go with with is ...
[わたし]{watashi}[は]{wa}[明日]{ashita}[寝]{ne}[る]{ru}.
OR
I plan to sleep tomorrow [interpreting go as a volitional construction]
in which case I would say
[わたし]{watashi}[は]{wa}[明日]{ashita}[寝]{ne}[る]{ru}[つもり]{tsumori}[です]{desu}.
Regarding the statement
For conversational Japanese, I need to add n desu for the sentence.
I am not sure where you are getting that rule, but at least for me (as a non-native speaker of Japanese), there are only a limited number of types of conversations where I would end sentences with [んです]{ndesu}.
One further place where I could be misunderstanding you which would affect the translation. Do you mean "I am going to sleep all-day tomorrow" or simply that your plans for tomorrow include sleep?