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Languid
Energy Levels
Use 'languid' to describe someone's temporary lack of physical or mental energy, not their overall personality. 
Avoid Negative
'Languid' can be used negatively, but it's less harsh than 'lazy', focusing on the current state. 
Not Always Illness
While 'languid' can imply illness, it's also used for a normal state of tiredness or relaxation. 