Scientists have been postulating for years as to why our energy levels often take a dip during the middle of the day.
And while some blame our biological clocks -- when it gets darker outside, our body temperatures drop and the sleep-inducing hormone melatonin is released -- others say carb-heavy lunchtime meals may be the cause.
In the study, which appeared online in the Dec. 15 issue of the Journal of Clinical Sleep Medicine, researchers found that a lack of vitamin D has an inverse association with how sleepy study participants were during the day. Among African-American participants, however, the opposite proved to be true, with sleepiness decreasing as vitamin D levels dropped.
Study author David McCarty, MD calls his findings complex. "It's important to now do a follow-up study and look deeper into this correlation," he said, noting that while sleepiness was present across all racial groups at the beginning of his study, it, along with vitamin D deficiency, was abnormally high among the African-American set.
The low levels of vitamin D were no surprise -- one third of African Americans are vitamin D deficient, according to the Centers of Disease Control and Prevention -- but from McCarty, its connection to daytime sleepiness was.
"It's the opposite of what I expected to find," McCarty told The Huffington Post. "I don't know why this funny association exists. It's not that low levels are better because these people are less sleepy. My theory is that as you progressively increase the crushing burden of vitamin D deficiency ... pain probably increases, markers of inflammation may go up," he said adding those symptoms could be what's keeping African Americans awake during the day.
"Maybe the overactive adrenaline and cortisol and all the things that go on with a stress response are acting as an increasing counterbalance to the sleepiness," McCarty said. "Those factors become more important the worse the vitamin D deficiency gets."
Looking deeper may require McCarty to expand his study pool beyond the 81 sleep clinic patients surveyed, an approach that researchers at the State University of New York (SUNY) took when they found a connection between sleep and race earlier this year.
In their study of some 400,000 respondents from the National Health Interview Surveys, SUNY doctors discovered that those born in the United States were more likely to report sleeping longer than the recommended seven to nine hours each night, with African-born Americans logging only six hours of sleep or less.
Similarly, sleep researchers at Northwestern University Feinberg School of Medicine in Chicago analyzed the sleep of 439 randomly selected Chicago men and women and found that white participants slept significantly longer than the other groups, while blacks reported the worst quality sleep.
But the study's co-author, Northwestern's Mercedes Carnethon, said her findings proved to be based on more social and cultural factors than physiologic ones.