Temporary Total Blindness when I Wake up - Help?

This has happened to me a couple of times recently, but most notably this morning. I awoke at 11:00 AM, so of course it was daylight outside. I turned to face the window and could see absolutely no light at all. Then my phone rang, so I picked it up but couldn't even see the light from the screen. Once I sat up, gradually the light became visible and I could see relatively normal, but I lay down again on my back and everything faded to black once again. My vision is very dark today, but having said that it is a very dark day outside. But naturally I found it very disturbing to be waking up in total darkness, and although this seems to correct itself once I'm upright, waking up in the black is disconcerting. Could it possibly be due to lying in a certain position? Problem with my neck or head/brain? Any ideas?