An article was published in "The New England Journal of Medicine" describing how you can get temporary vision loss, as found when getting the case history of a 22-year-old woman and 40-year-old woman.  The cause was pretty simple: lying down in bed on your side in the dark, looking at a smartphone, but only with one eye as the other eye is blocked by your pillow - when going back to using both eyes, the light-adapted eye (the one on the smartphone) went blind because it couldn't catch up to the other eye (the covered eye which was adapted to darkness) fast enough.  

The 22-year-old woman said she would lose her vision at night after several minutes on the smartphone, while the 40-year old said she would experience the vision loss when she work up in the morning.  The vision loss would last for up to 15 minutes.  Researchers are calling it "transient smartphone 'blindness'".  While it is known that vision loss can be a signal for stroke, this experience is harmless, researchers say.