Is Vitamin D 25-OH useful for sleep?

Whether vitamin D (25-OH) improves sleep is not clearly established. Vitamin D is crucial for bone health, immune function, and muscle metabolism. Some observational studies link low vitamin D levels with sleep disturbances (e.g., shorter sleep duration, poor quality). However, randomized controlled trials (RCTs) specifically showing a causal benefit of vitamin D supplementation for sleep are lacking. Measuring 25-OH vitamin D in blood is the standard test for vitamin D status, but testing solely for sleep issues is not indicated. Correcting a deficiency is sensible, but overdosing (toxicity) is possible and dangerous. Therefore, for sleep problems, first address other causes (stress, sleep hygiene, other deficiencies). Evidence: mixed (observational, no strong RCTs).