first light vs morning

first light and morning both are nouns.

Word	Noun	Adjective	Verb	Adverb
first light	Yes	No	No	No
morning	Yes	No	No	No

As nouns, first light and morning are synonyms defined as:

Other synonyms of first light include aurora, break of day, break of the day, cockcrow, dawn, dawning, daybreak, dayspring, sunrise, sunup.

first light (noun)	morning (noun)
the first light of day	a conventional expression of greeting or farewell, used to wish someone a good morning
	the earliest period
	the time period between dawn and noon
	the first light of day