I'm Dr. Stan, and will be assisting you.
After your doctor examined your eyes and concluded that there is not problem, meaning that your retina is intact and not detached. Here is more else that could be happening but general is not a concern until it becomes severe, and that is Posterior Vitreous Detachment (PVD), and also cause flashes.
Inside the eye is a clear gel that attach to and supports the back of the eye (retina). With age, this gel can start to melt and develop pockets of liquefied gel. The pockets of liquids inside the clear gel moves and can give the sensation of flashes, especially when it develops close to the back of the eye. Usually PVD is without any harm to the eye, except for the discomfort. However, when it becomes severe constant flashes and floaters, then the retina eye surgeon will removed the liquefied vitreous. Hopefully, the eye doctor who examined you was an ophthalmologist, who is better trained to carefully look at the back of the eye to exclude retinal detachment or severe PVD. If not, I would recommend that you see an ophthalmologist.
The other issue that need to be evaluated for and excluded is whether the flashes at night represent a break in the far periphery of the retina. If your eye doctor is an ophthalmologist, then I would request that you let him know that you are still very concerned about this, and request to see a retina eye surgeon. I trust that you will do well.
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Please note that this answer is for information only. It cannot be substituted for the visit to or the recommendation of your doctor.
Edited by Dr. Stan on 11/9/2009 at 11:46 PM EST