Sanity Meter

""Like many games with a Lovecraftian bent, it implements a sanity meter, as though sanity is like diesel oil or something, and you can get a reading on it by sticking a dipstick in your ear.""

- Ben "Yahtzee" Croshaw, Amnesia: Dark Descent review.

Some games will try to measure how bad or good you're being, on the basis that the game is set in a moral universe. This ain't one of those games. This is one of those games where the very nature of reality is mutable, there are things out there beyond human imagining that mean us ill, and you've encountered several of them first hand. After a while, that's really going to wear on a person...

Ergo, the Sanity Meter. Instead of measuring how good or bad you are, it measures how well you've managed to keep your mind together when facing the horrors from beyond reality's edge. Some games will actually merge the Sanity Meter and the Karma Meter, on the grounds that doing enough horrible things may either give you Post Traumatic Stress Disorder or disconnect you from the activity altogether. Either way, you'd better keep all your marbles clutched tight, 'cause it looks like Cthulhu's coming around again.

"Sanity Meter in video games" is, by the way, patented by Nintendo, the publishers of Eternal Darkness.

Tabletop Games

 * Call of Cthulhu (tabletop game) pretty much pioneered the Sanity Meter. Each character starts with some measure of Sanity out of 100. When encountering a Mythos horror, or something just plain horrific, they roll their Sanity score. If they succeed, they roll to see how many of a smaller amount of Sanity points they lose; if they fail, they roll to see how many of a larger amount of Sanity points they lose (e.g., "roll for 1d6/1d20 SAN"). SAN points can be regained through psychotherapy and successful adventuring, but learning more about the Mythos permanently decreases your maximum Sanity. In fact, it is absolutely impossible to learn everything about the mythos without your SAN falling to 0 before.
 * A form of Sanity Meter is actually implemented in the First-Person Shooter Call Of Cthulhu: Dark Corners of the Earth, where the longer you spend looking at (or just being around) the horror scenes, the more disturbed the protagonist becomes, eventually bringing in Interface Screw and, at one point, suicide.
 * In another take on Lovecraft RPGs, Trail Of Cthulhu, the Sanity Meter is broken down into two parts: Stability and Sanity. The former represents short-term sanity and explains how you can have characters who are nuttier than a bag of fruitcakes and yet still interact with society on a relatively normal level. Stability can regenerate, but once a character loses Sanity points, they're generally not getting them back.
 * Parodied in Toon the Cartoon Role Playing Game. One of the settings in the Tooniversal Tour Guide supplement is "Crawl of Catchooloo", where exposure to the eldritch but otherwise strait-laced minions of the Elderly Gods drives the 'toon PCs sane (since they're already crazy to begin with). Sane PCs become boring, unremarkable characters with pointlessly dull interests.
 * Arkham Horror, being Cthulhu Mythos The Board Game, also makes use of sanity rules. Seeing monsters and casting spells are the primary cause of sanity loss, but many encounters can also trigger it. Being reduced to zero sends you to Arkham Asylum (not that one) with some items lost. (Before long, you'll regain enough Sanity to leave ... but you'll be back. Oh yes. This game takes hours.)
 * Changeling: The Lost has a Clarity rating for its characters. Unlike the standard Karma Meters for the other World of Darkness games, Clarity is more a rating of how well a person's able to hold it together after being put through hell at the hands of The Fair Folk. Murder and theft count as sins, but so do taking psychotropic drugs and experiencing unexpected life changes.
 * There is also the fact that in all of the gamelines, when the Karma Meter decreases, a charecter has to roll to determine if they gain a Derangement. The Storyteller is advised to select a Derangement appropriate to whatever caused the Karma Meter to decrease.
 * Exalted has the Limit meter, which measures mental and emotional stress. Max it out, and it drops back down to zero...because you've just unleashed all that stress in an outburst of insanity that can last anywhere from a few hours to several months.
 * The Chimera rules in the Lunars book also work like this, only fused with a sort of mutation meter: if you undergo this break in the Wyld, and you're a Lunar without moonsilver tattoos, you gain a permanent point of Limit. By about five or six, Lunars with tattoos will try to kill you on sight. When it hits ten, your mind and body are both reduced to screaming madness.
 * Palladium's RPG system has a horror factor stat for how monsters affect your sanity, simply by looking at them.
 * The Dungeons & Dragons setting Ravenloft, based on Victorian horror, adds to the traditional saving throws of Fortitude, Reflex, and Will by adding saving throws for Fear, Horror, and Madness.
 * In second edition, when Ravenloft first became a full-fledged campaign setting, Fear, Horror, and Madness were added to the five saving throw categories. In third, they became extensions of Will saves.
 * The Madness Meters in Unknown Armies. There are five; Isolation, Helplessness, Violence, Self, and The Unnatural. Depending on how well you roll when confronted with triggers, you either fill them with a Failed notch, or a Hardened notch. The more Failed notches you gather, the more likely you are to break down crying when you experience a trigger; the more Hardened notches you gather, the more immune you become to the trigger (to the point where a character with all Hardened notches in their Violence meter becomes a sociopath who doesn't really see the problem with carving a guy's face off with a potato peeler, and is only vaguely aware that others might not feel like he does).
 * Vampire: The Masquerade has a Humanity meter for vampires, which the lower it becomes increases the chance that a character will Frenzy, causing them to lose all control and attack targets randomly (both enemy and friendly targets) until it fades away.
 * A character's Humanity stat in the table game did much more than it did in Bloodlines. The Humanity stat for vampires is curious in that it did double duty as Sanity Meter and Karma Meter, but does not stop you from picking up mental illnesses for other reasons. The Humanity Meter indicates what sort of wrong doing bothers a particular character, where a saintly person might feel guilty for selfish thoughts, most humans draw the line at theft, and a person low on the meter might be bothered by "Acts of Casual Perversion" or however it was phrased. Humanity kept the character from giving in to the monstrous barbarism ("Beast") of their undead state, but it didn't protect them from suffering OCD, schizophrenia, or other forms of mental illness. You could be a paranoid schizophrenic saint who had their "Beast" on quite a leash - as a starting character.
 * Low humanity vampires also found it excruciating to act in the day, had extreme difficulty in mimicking human physiology when it would help them (for example, warming your skin so a potential meal you were attempting to seduce wouldn't realize you were a corpse), and at Humanity Zero, a character would pretty much become a mad dog who had to be put down. Many princes of the Camarilla (the "less evil" faction) would put a vampire down before Humanity 0 just because they became so violent, inhuman, and Axe Crazy they would threaten The Masquerade just by existing.
 * The game also featured alternative moral systems which were horrifying and outright evil by pretty much anyone's standards, but kept the Beast at bay. Such characters dove headfirst into being monsters to avoid becoming berserk, mad killing machines. For examples, one path says be God's personal scourge, another mandates you should be a ruthless bastard with Chronic Backstabbing Syndrome, and a third suggests you become an emotionless killer and student of death. Characters on these paths used very dark Blue and Orange Morality to remain less insane than the beast.
 * Vampire The Dark Ages featured alternate paths than Humanity which would be similar to Humanity and served a similar purpose. For example, the Road of Heaven put religious duties first, human ethics second, but the overlap between them was very high. Such a character fought the Beast with their faith.
 * Warhammer Fantasy Roleplay and Dark Heresy both have Insanity Points, which measure just how unhinged you've gotten by your adventuring. Encountering a sanity-blasting scene makes you test your willpower, with failure leading to one or more automatic insanity points.
 * In Warhammer Fantasy Roleplay you need to make a willpower roll for every Insanity Point you gain above 5; failure on this roll or reaching 12 insanity points resets your points to 0 and gives you a major derangement that will cripple or render your character unplayable right off the bat. On average, a bad roller will get two-three insanity points per session; double that estimate for a magic user. Unless you have a Gold Wizard, a very lucky brain surgeon or a Priest of Shalliya amongst your list of friends, insanities and insanity points are incurable.
 * Dark Heresy has a 0-100 score on insanities: You automatically start getting some minor derangements once you get over 30, with the number and severity increasing for every 10 over that. Once you reach 100, your character is unplayable. Unlike in WFRP insanity can actually be a good thing as it makes you more resistant to fear: A character with more than 80 insanity points can stare down an Eldritch Abomination without much trouble, but at that point the voices in his head will already be doing far worse to him on a daily basis. On average, a session of Dark Heresy is about six-eight insanity points for an unlucky roller, with about two-three more per session for being or standing too close to a psyker. Insanity points can be bought off during reconciliation time for about 100 XP a point.
 * Only War has a similar (if not the same) mechanic as Dark Heresy.
 * The attempt to make a Wheel of Time RPG based on 3rd Edition Dungeons & Dragons rules had this for male channeler characters. As male channelers eventually go insane, each time the character gains a level or uses too much power, he has to roll a die to determine how much sanity he loses. There was also a table laying out what psychoses the characters should manifest at what levels of sanity.
 * While GURPS doesn't have a sanity meter in the basic game (though it's very easy to add one, considering the system), it does include Fear checks whenever a PC encounters something particularly or personally terrifying. The basic game includes a massive table of effects that can be caused by fear, from becoming somewhat shaky to falling into a coma.

Video Games

 * The titular character Alice from American McGee's Alice has two meters. Her Strength of Will is how much ass-kicking she can deliver while her Level of Sanity is how much more ass-kicking she can receive. She gets more sanity by drinking the life essence of creatures she's killed. Riiiight. Of course, when you're basically wandering around in your own head, and the creatures you're killing are literally your own inner demons made manifest, it makes a certain amount of sense that taking them down would improve your sanity.
 * Amnesia the Dark Descent has one of these, in much the same vein as Call of Cthulhu. Regarding the Interface Screw, it can range from a drunken haze to laying down and dragging yourself along while insects crawl along your field of vision, and paintings become grotesque.
 * Clock Tower 3 has a panic meter. If it fills, Alyssa starts running around uncontrollably. Cue Yakety Sax.
 * Haunting Ground has a very similar concept. If Fiona reaches "panic mode", she becomes disoriented and cannot be well controlled by the player (to the point where the entire screen goes monochrome and disables use of the inventory/item menu).
 * Clock Tower: The First Fear has this too, but it is slightly different in that it simply causes you to trip more frequently and makes traps more likely to kill you.
 * The Nightmares quality in Echo Bazaar is an inverted sanity meter - it starts at zero, and increases as you go through particularly horrific experiences. At eight you Go Mad from the Revelation. It can be brought back down through particularly soothing experiences, writing down some of your darker secrets in journals, or laudanum abuse.
 * Once you Go Mad from the Revelation, it can be easily brought back down through actions in the special location you're sent to, but once you become sane again, you will lose all of your dream progress, making it the most punishing of the four Menace locations.
 * Elona has a sanity stat. It is, however, mistranslated and actually a measure of insanity, as the healthiest possible level is 0. As it increases you are more vulnerable to effects that cause the insanity status which causes you to lose control of your character.
 * Eternal Darkness brings the Sanity Meter to video games. If it starts going low enough, you experience hallucinations; if it drains entirely, you take damage whenever a monster sees you. These hallucinations include Breaking the Fourth Wall, reciting Hamlet, healing spells that backfire and cut you in half, and most interestingly, Interface Screws such as game resets, cranking down the volume, switching to another video input, or "deleting" your savegames instead of saving.
 * Fahrenheit (2005 video game) has a Sanity Meter for each primary character. It's more about emotional stability than actual sanity, however; once it reaches zero, the character experiences a complete mental breakdown, and it's game over. It also functions comparably to a Karma Meter: carrying out morally suspect actions will cause it to drop, and vice versa.
 * Tyler gets one moment at the end of the game,
 * For an incredibly early (perhaps the earliest?) example, check out Domark's 1985 Friday the 13th on the Commodore 64, Amstrad and Spectrum (it's nothing like the NES version). The game has a sanity/fear meter that raises as the game goes on. It's represented as a kid's head, with the hair starting to stand on end as you get more frightened. If the meter maxes out, you die of fright... this almost never happens in-game, but it does increase the chances of seeing hallucinations (a pile of skulls covered in blood, someone with a machete through their head, etc) accompanied by a blood-curdling scream. Surprisingly effective for 16-colour graphics!
 * The Geneforge series has a very simple (and invisible) one. It increases every time the PC uses a canister. Use too many and people will start to notice it and react to the PC differently. The PC will also occasionally go into an uncontrollable rage and attack people.
 * Lusternia has one, which is gradually eroded by spending time on The Astral Plane or inside Muud. It causes the player to hallucinate, and cancels a lot of the commands you enter.
 * The Shadow Hearts series has Sanity Points (SP), which deplete by 1 with each combat round. Once it hits zero, the character goes Berserk, attacking randomly without player input; if they end a battle in Berserk status, they can't remember the events of it and thus gain no experience points. Harmonixers lose Sanity Points faster when they use their Fusion abilities (demons and/or spirits attack their sanity), but they start with about twice the SP of any other character.
 * The Thing gives your party members sanity meters. If a Thing tries to eat them, they won't react well. (Not uncommonly, one of them is a thing, and the others utterly freak out when they're betrayed.)
 * hair in Tsukihime is actually a sanity meter of sorts. The redder it gets, the closer she is to falling into madness from drawing too much upon her demonic powers. It's not actually until her eyes turn red that she loses it, however.
 * The battle with Yogg-Saron from World of Warcraft uses a sanity meter. Players start with 100, and Yogg has numerous abilities that reduce it. The only way to recover sanity is to stand in one of the green beams of light (sanity wells) situated around the room. If a player reaches 0 sanity, they go insane and begin to attack all the other players in the room until they are killed (the player will actually start seeing all his fellow players as Faceless Ones, servants of Yogg-saron). This not only hurts the group by losing a friendly player, but said player can also kill other players while insane.
 * On top of that, every so often players have to go inside Yogg-Saron's mind and attack his BRAIN while he's casting an induce madness spell. Failing to get out of Yogg-Saron's head in time will cause the people inside to instantly go insane.
 * Several variants of Angband implement a sanity readout, usually in the form of "Current Sanity/Max Sanity" due to the typically text-based nature of said games. Notable variants include Cthulu Band and Tales of Middle Earth, which borrows a number of elements from the former.

Choose Your Own Adventure
In the Fighting Fantasy book House of Hell you collect fear points when something eerie happens. If you get too much, you instantly die.

Real Life

 * Essentially Truth in Television, although the human version averts Critical Existence Failure, with effects (and percent chance of death) worsening the lower the meter goes. In addition to PTSD, brain damage and other types of insanity, the brain also tracks emotional distress over time and takes steps.