flatness vs sluggishness

flatness and sluggishness both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatness	Yes	No	No	No
sluggishness	Yes	No	No	No

As nouns, flatness and sluggishness are synonyms defined as:

flatness and sluggishness: inactivity; showing an unusual lack of energy

Other synonyms of flatness include languor, lethargy, phlegm.

flatness (noun)	sluggishness (noun)
the property of having two dimensions	inactivity; showing an unusual lack of energy
inactivity; showing an unusual lack of energy	the pace of things that move relatively slowly
the property of having little or no contrast; lacking highlights or gloss	a state of comatose torpor (as found in sleeping sickness)
a deficiency in flavor
a want of animation or brilliance