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FRIDAY, Dec. 14 (HealthDay News) -- A strong but complicated association exists between vitamin D levels and daytime sleepiness, and race is a major factor, according to a small new study.
The study included 81 sleep clinic patients who were eventually diagnosed with a sleep disorder. Most of the patients were found to have obstructive sleep apnea. People with obstructive sleep apnea experience repeated breathing interruptions while they sleep.
The patients' levels of daytime sleepiness were assessed and blood samples were taken to measure their vitamin D levels.
Among patients with normal vitamin D levels, progressively higher levels of daytime sleepiness were associated with progressively lower levels of vitamin D, the investigators found.
Among patients with vitamin D deficiency, an association between vitamin D levels and daytime sleepiness was seen only in black patients. But in these patients, higher vitamin D levels were associated with higher levels of daytime sleepiness, according to the study published online Dec. 15 in the Journal of Clinical Sleep Medicine.
"While we found a significant correlation between vitamin D and sleepiness, the relationship appears to be more complex than we had originally thought," principal investigator Dr. David McCarty said in a news release from the American Academy of Sleep Medicine.
"It's important to now do a follow-up study and look deeper into this correlation," he added.
This study is the first to demonstrate a significant association between vitamin D levels and sleepiness, according to the researchers. They said it makes sense that race would affect this relationship because darker skin is a known risk factor for low vitamin D levels. This is because the body makes vitamin D, sometimes called the "sunshine" vitamin, when the skin is directly exposed to the sun.
While the study found an association between daytime sleepiness and vitamin D blood levels, it did not prove a cause-and-effect relationship.
The U.S. Centers for Disease Control and Prevention has more about sleep and sleep disorders.