How to Stop Feeling Tired 24/7?

I am tired all the time, most of the time for no good reason! I get 8 hours sleep, do not work a mentally/physically demanding job, work normal hours and for some reason I come home completely exhausted. Wake up exhausted, even on weekends!

I know some of it has to do with my diet/exercise habits, I have slipped a lot. But even before when I was physically fit, same deal.

I for some reason just rarely have energy.

Any tips? Experiences?