A state of fatigue, either physical or mental slowness and sluggishness, with difficulties in initiating or performing simple tasks. Distinguished from apathy which implies indifference and a lack of desire or interest in the task. A person with lethargy may have the desire, but not the energy to engage in personal or socially relevant tasks. The phenotype is Lethargy (HP_0001254, a Human Phenotype Ontology entry). Also known as: Dullness, Inaction, Inactivity, Languor, Slowness, Torpor.