Why Do I Feel Sleepy And Tired All The Time.

Why do I feel sleepy and tired all the time. I wake up I have a cup of decaf coffee in the morning around 7:00am. I then get sleepy around 8:30am needing to go back to bed really suddenly to the point I have to lay down for 30min, although I don't feel any better I push myself through the day feeling sleepy all day. My husband notice I'm dragging,I'm not so talkative just fighting to stay awake. I hit these spurts every so often. I hate it. I do take vitamin D, and B12. I was going to actually allow myself to sleep, just to see if I would really sleep the whole day away....It doesn't feel healthy.