To add some "anecdata" to the discussion, vitamin d supplements changed my life. I learned that my vitamin d levels were super low (7 ng/ml, when less than 20 ng/ml is deficient) from a routine blood test. Within a month of taking vitamin d, my afternoon fatigue went away, it became much easier to wake up in the morning, my energy levels generally increased, and my mysterious colds that I was getting every 1-2 weeks that winter stopped.

I strongly recommend getting your vitamin d levels checked!