In younger adults, fatigue is often lifestyle-driven: irregular sleep, heavy screen time, low physical activity, poor protein/fiber intake, and academic/work stress. Common medical contributors include iron deficiency (especially with menstruation), low B12 (restrictive/vegan diets), low vitamin D, thyroid disorders, and anxiety/depression. If you exercise but tire easily, consider low ferritin or inadequate calorie/protein intake. Persistent fatigue can also follow infections or, more rarely, reflect chronic fatigue syndromes—so prolonged or worsening symptoms merit evaluation, especially with red flags.