I'm editing my post because I just read your reply in the thread that said you had been to the doctor.
If you are borderline anemic, that could also explain your tiredness. I too am anemic, and if I don't take my iron supplements (the RIGHT supplements), than I am always sleepy.
What supplements are you taking? I really like the liquid Floradix. It doesn't taste bad at all (it actually is growing on me) and you can chase it down with a glass of orange juice to help absorption into your body. I can't take any kind of pills or capsules because they don't work for me. Maybe trying a different supplement might benefit you as well?
Also, take a look at your diet. Are you eating regular meals (6 times a day -- small ones) and drinking your water? Are your meals healthy and nutritious? Do you do a cardio exercise 2-3 times a week? Eat figs, molasses, green leafy vegetables, almonds, beets, dates, celery, canteloupe, parsley, oatmeal, yogurt, lentils, the occasional piece of dark Chocolate. Try to eliminate sugars and sweet stuff. make sure you are eating the right kinds of fats and eliminate all saturated fats and trans fats.
If none of the above has pointed to a solution, then perhaps you just need to go to bed earlier. Maybe a 10:00 bedtime or 9:30 bedtime instead of 10:30 (I go to bed at 10, and get up at 7). Or try to take JUST a 30 minute nap in the afternoon.