Is Vitamin D 25-OH useful for Schlaf?

Measuring 25-OH vitamin D (calcidiol) is often linked to sleep quality, but the evidence is mixed. Some observational studies suggest low vitamin D levels are associated with sleep disturbances, shorter sleep duration, and daytime sleepiness. Mechanisms include vitamin D's role in melatonin regulation and anti-inflammatory pathways. However, robust randomized controlled trials proving a causal benefit of vitamin D supplementation for sleep are lacking. Most recommendations are based on correlational data or small interventions. Correcting a deficiency is advisable for general health, but as a standalone therapy for sleep issues, the evidence is weak. Caveat: single measurements are not diagnostic; sleep problems have many causes. Evidence: human-moderate.