Persistent fatigue can be lifestyle-related (poor sleep, stress, nutrition gaps) or reflect medical causes such as anemia/iron deficiency, B12/folate deficiency, thyroid disease, glucose disorders, chronic infections/inflammation, medication effects, or depression/anxiety. If rest doesn’t help, exercise tolerance drops, or red-flag symptoms appear (shortness of breath, palpitations, weight loss, fever, night sweats, bleeding), seek clinical evaluation. Some people experience prolonged post-infectious fatigue or chronic fatigue syndromes, which require exclusion of other causes and follow-up.