I feel tired all the time even when I get 8 hours of sleep. What is that?

I am so tired, tired. Sleep quality is best ascertained by asking yourself in the am, "am I rested? " if you move around alot in your sleep, snore/gasp, or seldom dream, you may have a sleep disorder. Persistent fatigue has innumerable causes including depression, seasonal affective disorder, medications, anemia, hypothryroidism, substance abuse, or any of a number underlying medical problems. Consult your md.
Anything. You can have sleep problems from snoring, apnea, restless legs, poor sleep hygeine, anxiety/depression, napping during the day, night seizures, noises in the neighborhood waking you up, bed bugs, waking up to use the bathroom, acid reflux, allergies, or any number of things. I recommend seeing a doctor to get checked out.