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Cave adaptation is a mechanic that causes dwarves who spend too much time underground with insufficient exposure to outdoor sunlight to grow sick when finally exposed to it. It is controlled by the
Exposure of cave-adapted dwarves to outdoor sunlight can cause two negative thoughts: "irritated by the sun" and "nauseated by the sun". The latter results from more severe cave adaptation and triggers profuse vomiting, which can leave an otherwise capable soldier momentarily defenseless. Cave adaptation causes no problems if the affected dwarfs simply remain underground.
 Prevention and treatment
First and foremost, be aware that cave adaptation requires much more effort to fully eradicate than it's worth. Most dwarfs can spend their entire lives lurking in the dark with no negative consequences. Cave adaptation is only a major concern for your military dwarfs, as they are the only ones who really need to be functioning at full capacity when outside. Menial labor dwarfs sent out to build roads or structures can endure the unpleasant aspects of cave adaptation just fine, while dwarfs you regularly send out to fish or gather plants will quickly lose their cave adaptation entirely. Also, note that only dwarves suffer from cave adaptation. If this process sounds like too much effort, you might just want to recruit a few non-dwarven mercenaries for outdoor combat.
Treatment of cave adaptation is accomplished by exposing your dwarfs to outdoor sunlight for an extended period of time. Any tile with the Outside and Light traits will suffice, gradually reducing the cave adaptation of any dwarf standing there. This can't be cheated - even seemingly transparent roofs made of grates or glass still cause the tile to be treated as Inside. The progress of treating severe cave adaptation can be observed through a dwarf's Thoughts and Preferences, as limited exposure to sunlight will first convert severe cave adaptation into mild cave adaptation; see personality traits for details.
It takes a lot of sunlight to undo any serious amount of adaptation; a fully cave-adapted dwarf requires 9 weeks of exposure to be fully cured. As such, you'll need to devise entertaining and well-decorated meeting areas to lure your dwarfs to the surface and keep them there. Make sure there are plenty of high-value decorations in place, or the negative thoughts associated with being in sunlight will cause your dwarfs a lot of stress. Also, be aware that any such zone is ripe for attack by invaders; make sure there are soldiers nearby, surround the site with high walls, and consider building a retractable bridge above it so you can quickly block out the outside world as needed.
For your military, a cheap and sloppy solution is to simply send them outdoors a lot. Attacking the local wildlife is a low-risk exercise that gives your soldiers valuable military training alongside their cave adaptation treatment. Unfortunately, this is not an automated process; letting your dwarves return to the darkness will gradually reestablish their adaptation, and micromanaging your dwarfs that much quickly becomes annoying. If you can prevent the reestablishment of cave adaptation, however, this process is much more effective.
Cave adaptation can be prevented by keeping your dwarfs out of Dark tiles as much as possible. Every moment spent in the dark slightly increases your dwarfs' cave adaptation, eradicating the benefits of your surface garden parties, but fortunately these tiles can be converted to harmless Light tiles with nothing more expensive than time, effort, and a large supply of rock blocks. Any tile that has had all its roofing removed becomes an Outside Light tile, the sort which is useful for treating cave adaptation. Building a roof over these tiles replaces the Outside trait with Inside, but leaves the Light trait untouched. These sunlit indoor tiles neither cure nor cause cave adaptation, making them perfect for housing those dwarfs who will be heading to the surface often. Once you have a sizable workforce and some well-trained miners, it shouldn't take too long to channel away the top few floors of your fortress and reconstruct them with blocks. Doing this for your entire fortress, however, is almost certainly more work than it's worth. If you have a dedicated squad of soldiers for surface threats and convert their training and living areas to Light tiles, they should be safe from cave adaptation going forward, especially if you are regularly treating them to wildlife hunts and missions abroad.
 Cave adaptation mechanics
- If a dwarf is in a Dark tile, cave adaptation increases by 1 every tick to a maximum of 800,000 (403,200 ticks is one year, so the maximum is just short of 2 years).
- If a dwarf is in an Outside tile, cave adaptation decreases by 10 every tick.
- If a dwarf is in an Outside tile while the sun is out (i.e. not raining or snowing), the following happens:
- If cave adaptation is between 403,200 and 604,800 (between 1 and 1.5 years), the dwarf will become dizzy and experience minor pain and fatigue. The dwarf will also receive the unhappy thought "was irritated by the sun recently".
- If cave adaptation is at 604,800 or higher (1.5 years), the dwarf will experience nausea, dizziness, pain, and fatigue. The dwarf will start vomiting profusely and will also receive the unhappy thought "was nauseated by the sun recently".
- Otherwise, nothing bad happens.
Negative effects of cave adaptation have a percentage chance to trigger (1/1000 per tick) and they are cumulative, so a sufficiently long string of unlucky rolls can actually cause a cave-adapted dwarf to pass out from exhaustion upon stepping outside. Note that it is possible to get both cave adaptation thoughts at the same time.
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A scientific explanation of cave adaptation by Deus Machina.
Underground has low levels of light and very little air circulation.
Dwarves develop low-light vision (plump helmets are packed with beta carotene!) and their facial follicles become sensitive to the motions that a breeze produces against their beards, which allows them to tell where tunnels turn.
As they go deeper, their eyes become less relied upon, and they adapt further to rely on their whisker-based follication.
These eventually become so sensitive that, should a dwarf venture outside, the wind is the equivalent of multicolored and varying strobe lights. This is as aggravating to a dwarf's follication as a Pink Floyd show seen while sober is to our sight, up to the point of causing nausea.
While some have proposed beard shaving as a method of treating hyperfollication, these people are believed to be elf spies, and drafted to cavern exploring instead.