sleeper vs somnambulist

sleeper and somnambulist both are nouns.

Word	Noun	Adjective	Verb	Adverb
sleeper	Yes	No	No	No
somnambulist	Yes	No	No	No

As nouns, somnambulist is a hyponym of sleeper; that is, somnambulist is a word with a more specific, narrower meaning than sleeper:

sleeper (noun)	somnambulist (noun)
an unexpected hit	someone who walks about in their sleep
tropical fish that resembles a goby and rests quietly on the bottom in shallow water
a passenger car that has berths for sleeping
one of the cross braces that support the rails on a railway track
a piece of furniture that can be opened up into a bed
pajamas with feet; worn by children
a rester who is sleeping
an unexpected achiever of success
a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal