I know exactly what you are talking about. Where it doesn't matter if you have 7 hours of sleep or 17, you just cannot seem to feel rested. I know for me the main issue was stress, especially stress at work. I would suggest looking at some pre-sleep stress removal meditations. Use the meditation to relieve the stress, then your mind will actually be able to rest while you are asleep.
But like the previous person said, a medical professional may need to be consulted if the issue becomes enough of a problem to affect your normal daily activities.