Insomnia is rarely explained by a single vitamin/mineral deficiency; stress, circadian rhythm, environment, and medical conditions interact. Still, some deficiencies may indirectly affect sleep. Iron deficiency is relevant especially through restless legs syndrome, which disrupts sleep with an urge to move the legs at night. Vitamin D has been linked in some research to sleep regulation pathways. Magnesium plays roles in neuromuscular function and relaxation, and certain clinical studies suggest benefit in selected insomnia complaints. Vitamin B12 is important for neurologic function and blood formation, but it is not a guaranteed insomnia “fix.” A practical approach is symptom- and risk-based testing. If leg restlessness, marked fatigue, hair loss, or palpitations accompany sleep problems, ferritin/iron stores may be assessed. Low sun exposure plus musculoskeletal symptoms can justify vitamin D evaluation. Avoid blind supplementation—measurement and individualized planning are safer. Evidence base: epidemiologic/genetic studies on micronutrients and sleep; clinical trials involving magnesium-based interventions; differential diagnosis frameworks for sleep disorders.