Vitamin D deficiency is a silent widespread issue for a lot of people. Had a battle with it a few years ago. Was tired no matter how much sleep I got, had negative motivation to do anything. Libido all but disappeared, and finally sought a doctor visit when I started have suicidal thoughts.
Turns out I just needed more sunshine and 50kiU once a week for a month.
Vitamin D toxicity is also a big problem, and doctors have been overprescribing supplementary vitamin D for years. Feels almost like a joke, you're damned if you do, damned if you don't.