What is Causing My Uneven Eyes?

My left eye is little bit higher than the right eye, the problem is when I am tired or didn't have a good sleep, the left eye would be even more higher than the right eye. I feel that the muscle above then left eye is never relaxed, it is always very tense, my left eyebrow is also higher than the right eyebrow. Would the muscle make an eye appear to be higher? When I am not tired and had a good sleep, the left eye sometimes appears to be even. What could be causing this uneven eyes? the muscle?