Geralt's vision will now automatically adjust when you walk into a dark place (or just walk outside at night), no cat potion or hotkey required. Effect can be disabled and adjusted in-game. For immersion, you could set the intensity to 50%, so Geralt will be able to see better in dark places, but drinking a cat potion will still enhance his vision further (100% intensity unless you have another mod). And yes, drinking a cat potion will override the effect, it will not break.

I am aware that a similar mod already exists, but the author says it is broken, and does not always remove the effect. My mod works differently. I have tested the troll cave where you meet Thaler (which was causing issues in the previously mentioned mod, as the effect would not go away after exiting the cave), and everything works as it should. Also, as previously mentioned, this mod is configurable. This is also the only cat effect related mod that accounts for dark places that aren't marked as dark places in the game.

Note: If it's not activating, that probably means you are not within a "dark place" according to the game. Unfortunately there isn't anything that can be done about that (as far as I know), unless the "dark place" zones can be edited. If you find any such areas, let me know in the forum topic Report Unmarked Dark Places, I can add them manually now.

Installation:
Just move the "Mods" and "bin" folders into your main Witcher 3 directory. You will most likely have to merge r4Player.ws. 
Make sure you go into the settings (Options->Mods->Natural Cat Vision) and enable the mod, as it will be disabled and have an intensity of 0 by default.

To use the hotkey that toggles the mod on/off, copy IK_NumPad0=(Action=ncvKey) to Documents\The Witcher 3\input.settings under every category you wish to use it in, so those would be-
[Boat]
[BoatPassenger]
[Combat]
[Diving]
[Exploration]
[Horse]
[Scene]
[Swimming]
You can replace NumPad0 with any key you like.

Update 1.8:
-NCV now checks every second, not every player tick, which should minimize any performance loss
-playerWitcher.ws and useVehicle.ws are no longer part of the mod, only modified vanilla script is r4Player.ws, which should now be easier to merge
-Added custom "dark place" to cave where the Temple of Lilvani and royal wyverns are found

Update 1.7:
-Reworked internals of the mod (should be easier to merge and easier for me to update)
-The optional file with playerInput.ws is no longer necessary for the hotkey (you should remove the old version if you had it)
-Toggling mod on/off with the hotkey shows notifications now, instead of messages
-Added custom "dark places":
Cave under Temple Isle
Est Tayiar (partially)

Update 1.6:
-Fixed cat vision not activating while on horse/in boat
-Fixed 8:00 PM not displaying properly in options

Update 1.5:
-Added option to set custom night hours and recommended settings preset

Update 1.4:
-Added option to disable effect during cutscenes/dialogue, fixed localization files

Update 1.3:
-Added hotkey for toggling the mod on/off (optional), some optimizations

Update 1.2:
-Added Russian localization and w3strings files for all languages (they are in English though)

Update 1.1:
-Taking an actual cat potion will override this mod (for real this time)
-Added night mode, can be disabled in settings and also has separate intensity slider. Effect will not be active when Geralt goes into an interior, even during the night. Going into a dark place at night will use the default intensity, not night mode intensity.

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