There are lots of reasons that may cause blurry vision. If you are very tired or lack of sleep, it may happen. If you have some eye diseases, it may also appear. Eye diseases like cataract, glaucoma, vitreoretinopathy, retinal or vein obstruction, chronic keratitis all may cause blurry vision. As your sudden blurry vision happens periodically, I think you should see an eye doctor, or some serious problem may get happen. Best wishes for you!
This is a normal phenomenon. Don't worry. Let me tell you why. A lot of organs including your eyes assume half dormancy position during all night. When you wake up, some kinds of organs will work normally and the blood of your head reduce relatively. So both of your eyes are blurry. Besides, during one night's sleeping, your eyes produce tear secretions and the tear secretions cover the surface of your eyeballs. But there is another possibility that you suffer anemia. You need to ask for your doctor to seek some help and get useful medical treatment.