Fatigue: Fatigue is similar to lethargy but typically refers to a general feeling of exhaustion or weariness without necessarily involving a lack of motivation.
Depression: Depression is a mood disorder characterized by persistent feelings of sadness, hopelessness, and loss of interest in activities. Lethargy can be one symptom experienced by individuals with depression.
Chronic fatigue syndrome (CFS): CFS is a complex disorder characterized by extreme fatigue that cannot be explained by any underlying medical condition. It differs from lethargy as it involves more severe and persistent fatigue that lasts for at least six months.