service vs watch night

service and watch night both are nouns.

Word	Noun	Adjective	Verb	Adverb
service	Yes	No	Yes	No
watch night	Yes	No	No	No

As nouns, watch night is a hyponym of service; that is, watch night is a word with a more specific, narrower meaning than service:

service (noun)	watch night (noun)
an act of help or assistance; something (such as a tool, software or system) used to render said help or assistance	a devotional service (especially on New Year's Eve)
the act of public worship following prescribed rules
(sports) a stroke that puts the ball in play
the act of mating by male animals
the act of delivering a writ or summons upon someone
periodic maintenance on a car or machine
the performance of duties by a waiter or servant
employment in or work for another
work done by one person or group that benefits another
(law) the acts performed by an English feudal tenant for the benefit of his lord which formed the consideration for the property granted to him
tableware consisting of a complete set of articles (silver or dishware) for use at table
a means of serving
a force that is a branch of the armed forces
a company or agency that performs a public service; subject to government regulation