SAD lights won't affect your vitamin D levels ... they emit the wrong type of the UV light spectrum to generate vitamin D. However, I have tried both. Vitamin D3 supplements definitely helps me reduce inflammation. And the SAD lights do perk me up. I don't get depression related to SAD. But I do suffer from fatigue and idiopathic hypersomnia, which the SAD lights do help. The lights perk me up and I don't get tired until much later in the day.