One of the most easily observed conditions that happens during the sleep transition are hypnagogic hallucinations. These visual disturbances are a link between the full narratives of dreams and the waking world, and often take the form of abstract shapes, usually only seen for a few seconds. One of the most interesting elements of these sensations is the “Tetris Effect,” where people who spent time doing a repetitive activity before sleep experience hallucinations related to that activity – the titular example is seeing falling Tetris bricks in your peripheral vision as you’re trying to sleep. These sensations can be very convincing, and since the half-awake brain can’t rationally process them it’s easy to interpret them as real events.
Photo credit: Flickr user martinb via Creative Commons.