The two possibilities are migraine aura or tugging on the retina (the nerve layer that lines the inside wall of the eye). If the ring of light seems to throb, expand and contract, change shape, develop a "vapor trail" or change colors then almost certainly it is a migraine aura. If it just seems to be an amorphous glowing light that is always in the same spot, it could be retina trouble. If the latter, you should be evaluated by an Eye MD promptly.