lethargy vs sluggishness

lethargy and sluggishness both are nouns.

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

As nouns, lethargy and sluggishness are synonyms defined as:

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

Other synonyms of lethargy include flatness, languor, phlegm.

As nouns, lethargy and sluggishness are synonyms defined as:

lethargy and sluggishness: a state of comatose torpor (as found in sleeping sickness)

Other synonyms of lethargy include lassitude.

lethargy (noun)	sluggishness (noun)
weakness characterized by a lack of vitality or energy	inactivity; showing an unusual lack of energy
inactivity; showing an unusual lack of energy	the pace of things that move relatively slowly
a state of comatose torpor (as found in sleeping sickness)	a state of comatose torpor (as found in sleeping sickness)