This deficiency profoundly impacts cellular energy production, as vitamin D is crucial for functioning of mitochondria, the powerhouse of cells. “I sleep a full eight hours, I go for a run in the evening after work, I eat clean but come afternoon, I feel sapped out, exhausted and feel sleepy. This is a common question asked by many of my patients who are not aware of a key deficiency. Such energy crashes happen when you have low vitamin D levels. Her vitamin D levels were 12 ng/mL, far below the optimal range of 20-50 ng/mL.