I had a vitamin D deficiency which robbed my energy for several years. Since I started taking 1,000 IU Vitamin D every day, I have had much more energy and my "bad" days almost disappeared. This changed my life. I will take Vitamin D supplements for the rest of my life.
https://www.amazon.com/dp/B004U3Y8OM
This is an easy thing to try. You can pick up "Nature Made Vitamin D3 1000 IU" at Walgreens tonight.