Many players of The Witness have reported headaches and/or nausea during long game sessions. It has been agreed upon by many members of the community that this is caused by the game's low field of view (FoV), which essentially means that you have a narrow view on the screen, a large contrast from many modern first-person games.
While you can't fix this on the PS4 version, it is easily remedied on PC. Simply follow the steps below.
1. Locate your Local.variables file. This should be found under The Witness' data folder (C:Program Files (x86)SteamSteamAppscommonThe Witnessdata).
2. Right click Local.variables and open with a text editor such as Notepad or Notepad++.
3. Under the ":/misc" section, add the following: fov_vertical [NUMBER].
The default field of view is 54. If you want to increase the field of view, simply put a number higher than 54. 59 is recommended by many players, and would be input as "fov_vertical 59".
4. Save the file and re-open your game.
Note that although the line of code only states the vertical field of view, the horizontal field of view is calculated automatically based upon your aspect ratio. In other words, you don't need to enter a horizontal field of view.