There are a variety of reasons that people sleep as much as you are. From depression and hypothyroidism to chronic fatigue syndrome and sleep apnea, causes include a broad range of physical and mental health issues. Also bear in mind that some people simply need to sleep more than others. Talk to your doctor about this; some testing, including blood work, urinalysis, or even a sleep study may be helpful.
Many studies have always emphasized the importance and health benefits of getting enough sleep at night. Experts recommend 8-10 hours of sleep. This could contribute positively to a person's general well-being. But oversleeping could affect a person negatively, though. This could make one feel sluggish and fatigued
Waking up tired after a long night's sleep could be a signal of many things. Do you know if you snore? Snoring could sometimes cause a person's airway to close up for a few moments resulting in a lack of oxygen to the lungs causing you to gasp and struggle for breath and wake up for a second. Then you go back to sleep without even realizing that it happened. This could happen repeatedly all night resulting in fatigue in the morning. There is a test that a Sleep Disorders Specialist can perform to confirm if this is happening to you.
I think going to your Doctor and discussing this concern will be the best course. He/She can perform simple blood tests to look for possible causes. It could range from a simple anemia (a well balanced diet and iron supplement could fix this) to depression. You might be apprehensive about the results but being tired all the time could lead to more problems in the long run. In the meantime, maybe cut your hours to 8-10 hours a night as an experiment and see if there's a difference. Good Luck.
IM NOT TOO SURE ABOUT THIS SUBJECT SO I'M ONLY GUESSING HERE.....MAYBE LOW IRON OR YOU MAY BE MISSING SOMETHING IN YOUR DIET OR MAY NEED MORE VITAMINS?