What role does Vitamin D 25-OH play in sleep?

Vitamin D 25-OH (calcidiol) is the circulating storage form measured in blood to assess vitamin D status. It may influence sleep through vitamin D receptors (VDR) expressed in brain regions involved in sleep regulation, such as the hypothalamus. Observational studies have linked low 25-OH levels to shorter sleep duration, poorer sleep quality, and a higher prevalence of insomnia. However, randomized controlled trials (RCTs) supplementing vitamin D show mixed results: some report improvements in sleep parameters, while others find no significant effect. Thus, the evidence is moderate but not robustly causal. Consumer DNA tests (e.g., MyBody-X) can detect VDR gene variants, but these have limited predictive value for actual vitamin D status or sleep outcomes; blood testing of 25-OH is more reliable. Safety note: excessive vitamin D intake can lead to toxicity (hypercalcemia). In summary, vitamin D 25-OH may play a supportive role in sleep, but it is not a standalone treatment for sleep disorders and should be interpreted within a broader clinical context.