Dr. David E. McCarty of the Louisiana State University Health Sciences Center in Shreveport and colleagues at the Louisiana State University said the study was not designed to examine causality. However, the authors' previous and current research suggested suboptimal levels of vitamin D might cause or contribute to excessive daytime sleepiness, either directly or by means of chronic pain.
"While we found a significant correlation between vitamin D and sleepiness, the relationship appears to be more complex than we had originally thought," McCarty, the study's principal investigator, said in a statement. "It's important to now do a follow-up study and look deeper into this correlation."
The study involved a consecutive series of 81 sleep clinic patients who complained of sleep problems and non-specific pain. All patients eventually were diagnosed with a sleep disorder, which in the majority of cases was obstructive sleep apnea. Vitamin D level was measured by blood sampling, and sleepiness was determined using the Epworth Sleepiness Scale.
The study, published in the Journal of Clinical Sleep Medicine, found in patients with normal vitamin D levels, progressively higher levels of daytime sleepiness were correlated inversely with progressively lower levels of vitamin D.
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