It could be sleep apnoea that is causing the problem. This is when the airway is compromised during sleep, which reduces the amount of oxygen that reaches the bloodstream and therefore the brain.
The symptoms of this is waking feeling very sluggish and taking a while to 'come to'. After an hour or so, as the oxygen level normalises after waking, you will feel 'normal' again.
The longer you sleep in fact, the worse you'll feel as the longer you will have gone with a low oxygen level. This condition has a negative long term affect on the cardio-vascular system.
There are tests that can be done to establish if sleep apnoea is the problem. Sleep studies can be performed that measure your oxygen level during sleep. Treatments are available which include using an oxygen mask that delivers oxygen under pressure as you sleep. Speak to your doctor.