You’re tired all the time
If you aren’t getting enough vitamin D, you may feel completely exhausted, even if you get plenty of sleep. “There is mounting evidence that vitamin D deficiencies are associated with fatigue and sleep disorders,” says Catherine G. R. Jackson, PhD, a professor of kinesiology and exercise science at California State University in Fresno. A study in the North American Journal of Medical Sciences found that people who felt tired had low levels of vitamin D, but raising their vitamin D to normal levels significantly reduced feelings of fatigue. Not the problem for you? You may have one of these other 13 medical conditions for feeling tired all the time.

You’re depressed
Having the blues may be linked to an insufficient amount of vitamin D. According to the Vitamin D Council, vitamin D receptors have been found in many parts of the brain, including in areas linked to depression. Results from studies have been mixed—some researchers have found significant improvements in mood after supplementing with D, while others have not—but that could depend on the severity of the depression as well as the vitamin D deficiency. For example, researchers from Columbia University found that taking vitamin D supplements was effective for those who sufferred from clinically significant depression.