What Causes Vision Grey Out?

Twice in the past my vision has gone completely white. It was two different situations but, in both, I had had a definite lack of sleep the night before and I may have possibly been a little dehydrated. However, lacking sleep was by no means uncommon for me (average of 4-5 hrs unfortunately), and I have never drunk a lot of water (I know I should drink more). Both times I managed to acquire water, and then when I lowered the elevation of my head for a little while, it all went away, and I felt fine again. On one occasion I found out that by looking directly into a light, some of my vision would temporarily come back. The was no pain, no nausea, and I could not panic--only stay calm. I could think and maneuver (blindly) and hear fine. The second time I ran my head into a wall (could see it) and couldn't feel it. If it might factor in any, I rarely drink caffeine. Any idea what was going on? It's happened only twice, about a year apart. Oh, and it occurred in my mid teens.