I still feel tired after 9 hours of sleep. why does this happen?

Various reasons: You may have anemia, or low thyroid, perhaps a chronic fatigue syndrome. Also, a sleep disorder such as sleep apnea, may be present but not pinned down, (do you snore a lot?). Suggest that your doctor and you work this out and find a solution.

Sleep apnea: This may be because you are not getting enough oxygen during your sleep. This can be caused by sleep apnea , which can be obstructive ( do to the airways being blocked) or central ( no signal to breath. Uars , upper airway resistance syndrome is another possibility. Discuss this with your physician, as chronic lack of oxygen can cause strokes or heart attacks or other severe problems.