From what I've read optimal range is 65-80ng/ml. Studies done by the vitamin D council found that when levels were above 65ng/ml it was enough to get you thru the fall and winter without sunshine to keep your levels up, if memory serves me. Levels at 50ng/ml would drop during low sunlight seasons. A headache doctor (has talk on youtube) discovered the sweet zone (65-80ng/ml) with her headache patients. Once their levels got to that sweet zone, their headaches went away and so did their sleep apnea.