https://www.faasafety.gov/files/notices/2014/Dec/SA17_Spatial_Disorientation.pdf

Spatial Disorientation: What Is It? Spatial disorientation is the mistaken perception of one’s position and motion relative to the earth. Any condition that deprives the pilot of natural, visual references to maintain orientation, such as clouds, fog, haze, darkness, terrain or sky backgrounds with indistinct contrast (such as arctic whiteout or clear, moonless skies over water) can rapidly cause spatial disorientation. Pilots can compensate by learning to fly by reference to their instruments. But a malfunction of flight instruments, such as a vacuum failure, in conditions of reduced visibility can also end in spatial disorientation, with the same lethal results. While the physiology and dangers of spatial disorientation are taught during primary and instrument flight training, general aviation pilots still have misunderstandings about what it is and how to deal with it. And the accidents it causes continue to claim the lives of too many pilots and passengers every year.

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