What Causes Ringing In Ears While Asleep?

If I am trying to take a nap, or while I m lying down before actually falling asleep, once in a while I will hear this quick buzzing in my ears, see this rush of white light zoom into view (with eyes closed (like in a dream state) ) and then feel an electrical impulse or shock and it brings me back to total consciousness. [All of these things occur simultaneously and within one second]. With my eyes open now I feel perfectly fine, although it is very startling I am fully aware of what had just occurred. Its not like when someone is dreaming and falling within a dream and just before they hit something they ll get a jolt, and wake. I haven t yet fallen asleep. After this occurs like I said I feel fine, my breathing is normal, my heart rate is normal but I m just so perplexed as to what causes this. Help?