sinking vs lessening

sinking and lessening both are nouns.

Word	Noun	Adjective	Verb	Adverb
sinking	Yes	No	No	No
lessening	Yes	No	No	No

As nouns, lessening is a hypernym of sinking; that is, lessening is a word with a broader meaning than sinking:

Other hypernyms of sinking include decrease, drop-off.

sinking (noun)	lessening (noun)
a descent as through liquid (especially through water)	a change downward
a slow fall or decline (as for lack of strength)
a feeling caused by uneasiness or apprehension