"How's it going in there, Kyle? The first six months I was in solitary, I did push-ups every day and I never talked to myself. The next six months, I stopped doing push-ups and I... I confess... I did talk a little to myself. The six months after that... those next six months, Kyle? [Beat] You don’t wanna know what happened then."
One Piece's Brook spent fifty-plus years in total isolation and flashbacks suggest he definitely went at least a little crazy in that time. This might also have caused a deterioration of social skills that has resulted in Brook being one of anime's few post-mortem Dirty Old Men.
Made worse by his Devil Fruit powers, which resurrected his soul into his undying skeleton, so in his isolation he couldn't even look forward to dying of starvation or thirst before his second lifespan finally ran out at some unknown point in the future. Due to a promise he made he couldn't kill himself, either. So he was stuck there, alone, for fifty years, with only his instruments to keep him company — and the skeletal remains of his former crew, who he had been in charge of when they died.
Also Level 6 of Impel Down is Isolation for prisoners. Shiliew even stated that he was so bored he wished for death.
The Level 6 prisoners aren't completely isolated, being able to talk to each other.
Rozen Maiden manga has Kirakishou who's been isolated in the N-Field so long she begins to go insane.
Elfen Lied: Subverted by Mariko Kurama. She was immobilized in a giant containment unit soon after she was born and stayed there for 8 years, with her only contact with the outside world being a lab assistant speaking to her via an intercom. She isn't exactly a poster child for mental health, being a sadistic brat, but she eventually reveals herself to be far more bitter than crazy. And realistically, she should barely be able to talk, much less function on her own.
One of the reasons why Yugi from Tenchi in Tokyo turned to a life of villainy.
Likely the main reason Lucia from Rave Master is so screwed up. In general, locking a small child up in a maximum security prison and depriving them of contact with the rest of the world for ten years is bad for one's mental health.
Arguably, this trope is the story of Neon Genesis Evangelion in a nutshell. Every important character is completely alone (although not in terms of physical isolation). Although at the same time, forming relationships with other people is painful, kicking off the Hedgehog's Dilemma problem.
After an untold amount of time floating alone in space, a villain Cars, Big Bad of the second arc of Jojos Bizarre Adventure, is said to go so mad that his mind just shuts off.
The same thing happens to Magenta Magenta in Part 7, minus the space part.
Almost happens to Cyborg 009 in an episode of the 2001 series, when he spends a whole episode stuck in his Super Speed mode. What feels as mere seconds for the others, is for him several days spent unable to interact with anyone since he moves so fast that no one can see/hear/etc. him, and he can't touch them either as the smallest touch could set anything ablaze. It truly is an And I Must Scream situation, and he almost crosses the Despair Event Horizon.
Averted with the old man in Suicide Island. He's far more concerned with survival than contact with others — going so far as to actively avoid contact with anyone — but his mental health is possibly the best on the entire island.
Happens briefly to Hachimaki in episode 16 of Planetes. After he's inadvertently isolated for a brief time in space, he develops Acute Spatial Disorder and has problems doing his job in space. He's treated by being put inside a sensory deprivation room. Once it goes dark inside, his mind starts to play tricks on him, and he's forced to overcome the problem.
Element Lad of the Legion of Super-Heroes spent billions of years as the only being in the universe after being flung outside time and space in Legion Lost. He was driven very much insane as a result, although there was also some Showing Off The Perilous Power Source involved. It took him weeks to even remember his former friends when they were brought to his attention.
Inverted in DV8 #5, when Copycat gets trapped in a White Void Room. She's already mad (she has multiple personality disorder). Spending time in the void allows her personalities to start integrating.
Cable's recent series involving Time Travel and Bishop chasing after a MacGuffin Girl had Deadpool. Wait, I see you all there in TV trope land thinking "But he's already Crazy Awesome." but this Deadpool got stuck in a container underneath the earth for hundreds of years. He created another personality to play Hangman against and then started arguing with that personality cause it was better at playing Hangman than he was.
There's a comic in the Star Wars Expanded Universe, Mostly Automatic, which has a young man with a sweetheart taking a load of cargo on a trip which should have taken two weeks, during which he happily planned to lounge around playing games and watching vids. But a rock hit his ship, taking out the hyperdrive and the comm. Sublight engines still functioned, but it was ten parsecs to any kind of civilization and would take sixty years, alone on a little ship. He put the ship on automatic and then "quietly, and very deliberately... went... out... of... my... mind..." For the first few years he mostly slept until he ran out of sleep-inducing medication, then he went mad until he found an inactive service droid in a box in the hold and activated her, which helped.
Appa Ali Apsa, also known as the Old Timer, was once one of the Guardians of the Universe; in fact, he was the last Guardian to remain behind when the others departed for another dimension. Unfortunately, being left alone on Oa was not conducive to his continued sanity. He now had all the power of Oa too.
During the Obsidian Age story arc of JLA, Plastic Man is frozen, shattered into tiny pieces and scattered across the Atlantic Ocean floor almost 3000 years in the past. When the League finally reconstitutes him in the present, he reveals he was awake and aware the entiretime. During that period, he went insane and then became sane again because it got boring.
For an inversion, a later story involves the Flash curing a persistent malignant computer program called the Construct by duplicating it so that it had something similar to itself to talk to. He later discovers that the Constructs have since evolved into an entire species of electronic beings who worship him as a god.
During The Death of Superman, it's revealed that this is what happened to Hank Henshaw, the Cyborg Superman - after making his escape from Earth using a piece of Superman's birthing matrix rocket ship, he wandered the galaxy for so long, he went completely mad, having him believe that Superman drove him from Earth.
In the Silver Age Batman story "Robin Dies at Dawn", Batman begins hallucinating his sidekick's death after being locked in a sensory deprivation chamber for several hours.
Fanfiction author Asidian creates a scenario in their work Broken Glass to Sweep Away as a Rise of the Guardians work featuring Jack Frost and Pitch Black when the latter imprisons the younger spirit in a cage in his lair for an unknown period of time. It is heavily suggested the period of time is a few decades, and in the beginning Pitch suggests he would keep Jack here "80 years or so".
Griffin: I've made mistakes, Christ, we all 'ave. We tried to play at bein' God. It was arrogant and foolish and we thought we succeeded. We forgot that God is lonely. Why else would he have made man? Its not the age or the grief or the rage. Its the loneliness that has driven all of us mad. We cling to the people around us, to save us from ourselves. And when they die, we can't follow. After a while, the options become either cut yourself off totally or go completely insane.
Kasumi: "No one should ever be truly alone, Alex. Otherwise how would we stay sane?"
Films — Animated
Believe it or not, this actually happens in Happy Feet.
It seems that Rango had reached this point in the beginning where he sees each inanimate object in his glass box having a name and a personality and he is able to hear them talking. It's justified because Rango probably spent his whole life stuck in that cage without anybody to talk to but himself.
In Cast Away, our hero ends up talking to a volleyball... which, according to the survival specialist consultants on the film, saved his life from madness. In the script, Wilson even has "lines" which we don't hear.
Moon: Starts happening to the protagonist — Or does it?.
Rocket Man plays this for laughs when the protagonist is accidentally prevented from entering suspended animation for the nine-month trip to Mars, with clips of him at one day in, one month in, and a month in for each subsequent month. By the seventh, he's painting a replica of the Sistine Chapel's ceiling on the ceiling of the spaceship.
And at the end, it happens again...
Taxi Driver: Travis Bickle goes insane from the almost total isolation he experiences. He works and interacts with other people but he finds himself completely unable to connect to anyone and develops murderous tendencies. Most of his time is spent alone in his apartment or driving a cab.
Inception: This is one of the dangers of entering someone else's dream. Normally getting killed in a dream only wakes you up. However, if you're too deeply sedated, you wind up in a world where time passes much, much,muchfaster than normal. You will eventually wake up, but the question is, will you still retain your sanity?
Henri Young in Murder in the First spends three years in solitary confinement after attempting to escape from Alcatraz. He does have some human contact during those years; unfortunately, the humans are very sadistic guards. When he's finally released from solitary, he has a psychotic episode and kills the inmate who snitched on him and foiled the escape attempt.
Similar to the Stargate example below, when Jack Sparrow spends several months in Davy Jones's Locker between the second and third Pirates of the Caribbean films, he goes...well...even crazier than he was before. When the other characters arrive to rescue him, he assumes that they're just a more varied sort of hallucination (the ones he was having before were just lots and lots of iterations of himself).
In Sunshine, Captain Pinbacker was left alone in the Icarus I for 7 years, until the Icarus II comes along. He mistakes Capa for an angel. Then again, he was a little mad in the first place...
Oh Dae-Su from Oldboy gets locked up in room for reasons unknown to him for 15 years, being released when he was going to escape. He gets obsessed with revenge at any cost.
In the British film The Mindbenders a scientist turns traitor and then commits suicide when about to be arrested. An assistant and friend seems to know what was going on and volunteers to show them. They were experimenting with sensory deprivation which made the older man open to suggestion like self brainwashing. The younger man almost succumbs himself.
Mission to Mars finds Luke Graham marooned on Mars for a year after his crewmates are killed in a storm, and he attacks his rescuers when they arrive because he thinks they're just a hallucination. He quickly reverts to normal once he realizes they're real, however.
The Incredible Burt Wonderstone has Burt and Anton try to pull off a stunt where they stay in a levitated box in the Las Vegas heat for a week. They only last twenty minutes before Burt starts to freak.
Ben Gunn from Treasure Island is semi-insane from being marooned on the island for several years. He's coherent enough to help the heroes, though.
The Seventh Tower: Comes up as a problem for Tal when dealing with a character isolated inside a sunstone, complete with her spiritshadow. Considering the character herself admits to having been mad, Tal is wary in trusting her advice.
In the Star TrekQ Continuum novel trilogy, the omnipotent being 0 has spent millions of years isolated outside the galaxy (and his inability to travel at light speed precluded him from travelling to distant galaxies), and has turned mad from the isolation, making him even more powerful than the omnipotent Q.
In Jack Vance's Lyonesse, King Casmir imprisons Prince Aillas at the bottom of an oubliette. Aillas gradually loses his sanity and starts thinking of the skeletons of former inmates as friends and comrades in adversity. He gets better after escaping.
Marion Zimmer Bradley's story Elbow Room is something of a twist: the woman chosen for duty on an isolated station is actually all alone; the other people she thinks are there are her other personalities. She briefly flips out when she realizes this, but then goes back to the way things were. (Someone else who's read this story could probably describe it better.) There's a brief mention of how they tried sending groups of extroverts to man the station together, but they couldn't stand being cooped up together.
In the Firekeeper saga, the spellcaster Virim went mad from spending nearly a century or more alone in a tower far from civilization. When Firekeeper and her allies enter it, they find it full of various illusions and images of Virim constantly debating and arguing, representing his every second thought since unleashing the plague that killed the world's magic users.
In the second book in The Bartimaeus Trilogy (The Golem's Eye), Honorius is an example of this after being cooped up in Gladstone's tomb for over a century.
In James Swallow's Warhammer 40,000 novel Black Tide, Tarikus recounts how Fabius Bile keeps the captive Space Marines isolated for months or years to break them. In the short story "The Returned", Tarikus wakes with a jolt, taking seconds to realize he is no longer Bile's prisoner; he has suffered it since his escape. The psyker examining him comments off-hand that obviously he was deeply tormented by his experience. Later, after a test which rendered him just short of brain dead, he awakes peacefully and deeply relishes it.
Weirdly averted in most of the stories written by Larry Niven. He seems to assume that humans are able to survive extremely long periods of isolation without going nuts, as seen in situations such as people traveling through deep space for years, or a man with a time-accelerating device camping out inside it for six or more months so his arm transplant will heal and throw off the forensic investigators looking for someone who just got a new arm transplant. Played straight, however, in one short story by a guy who ends up spending several million years all alone doing the same routine while flying a spaceship. He loses the ability to think or act outside of that routine.
Total sensory deprivation and isolation is used as an interrogation technique by the KGB in Tom Clancy's The Cardinal of the Kremlin; one of the interrogators mentions that it's much more effective than torture. The specifics involve a neutral-buoyancy pool, carefully crafted restraints designed not to be felt, and a sound-isolation technique designed to neutralize the sound of the subject's own voice, so that the subjects couldn't even hear themselves talking.
Drizzt Do'Urden in R. A. Salvatore's Exile has a theoretically even worse problem - not only is he being affected by the isolation when hiding in underground caverns alone, but the "company" of the local Everything Trying to Kill You is causing him to combine this with reflexive killer's instincts that can pop up at the wrong time. The only thing that's keeping him somewhat sane through all this is talking to his cat. (Admittedly, she's a magical panther that can understand him, if not answer.)
The protagonist of House of the Scorpion suffers from a mild case of this, on account of being locked in a room full of chicken litter for six months.
The Stephen King short story "The Jaunt" had a futuristic mode of transportation which got people to their destination almost instantly, but they have to be knocked out beforehand. Otherwise, the person's mind feels like it spent an eternity in isolation. Anyone conscious during the trip arrives insane or just falls over dead.
I'll also mention that the Canadian, at the end of his strength and patience, made no further appearances. Conseil couldn't coax a single word out of him and feared that, in a fit of delirium while under the sway of a ghastly homesickness, Ned would kill himself. So he kept a devoted watch on his friend every instant.
The Saga of the Noble Dead has the ancient vampire Li'kan, who has spent thousands of years alone in an ice-covered fortress on a mountain peak, her unnatural life sustained by an Artifact of Doom. By the time the protagonists encounter her, she has forgotten even the sound of speech.
In Jack Campbell's Fearless, several rescued prisoners, despite each other's company, still were badly affected enough to wake up thinking they are back there.
The Lord of the Rings: Apart from the corrupting influence of the One Ring, living for several centuries in the darkness of a subterranean lake under the Misty Montains probably didn't help Sméagol/Gollum keeping his sanity.
In Remnants,Mother is a Sapient Ship whose creators abandoned her for unknown reasons, leaving her AI running. How does a computer go mad? Very, very slowly.
In Zel, a retelling of "Rapunzel", being trapped in a tower for two years with only the company of her mother for an hour every day causes Zel's sanity to slip to the point where she's walking around naked and babbling to a hallucination of a horse in her room.
Sail with Pirates: The pilot from Concepcion, after causing the ship to be wrecked at sea, runs away as soon the crew lands on an uninhabited island, and hides. He lives there alone for a long time. By the time the protagonist meets with him, he has become completely insane, but still gives some cryptic hints regarding the shipwreck location.
In the first book of Venus Prime series, Sparta is described as struggling to avoid crying herself to sleep during the years that she spends training to become a Space Board Inspector, because she's horribly lonely but also afraid that socializing might cause her to expose something about her past. In the fourth book, she suffers a nervous breakdown after a combination of prolonged isolation from others and a steadily-worsening drug habit.
Live Action TV
Buffy the Vampire Slayer: Wishverse Buffy from Season 3 is an even more by-the-book Slayer than Kendra, perhaps second only to the First Slayer.
In Doctor Who, isolation, or at least having no one to talk to to act as his conscience tends to do very bad things to the Doctor. This is especially proven in the new series, where he goes on a power trip and almost becomes the Master Mk. II in one episode because he didn't have a companion on hand to call him on his darker tendencies. He also admits he gets very lonely without someone around and the times he is seen alone he gets noticeably unhinged if the time elapsed is long enough.
In the audio drama Davros, the titular villain is rescued from a form of suspended animation. He explains to the Doctor that he was conscious the entire time, and went cycling between insanity and sanity about once every second. Ultimately subverted: Davros is as sane as ever once he's finally released.
The episode "The Girl Who Waited" has Amy trapped in a kindness facility for 36 years. She names her re-programmed robot Rory, and speaks to it.
Rory in "The Doctor's Wife", though that turned out to be the Genius Loci villain screwing with Amy. In a different episode, the real Rory spent 2000 years as a plastic cyborg and kept his sanity throughout. Possibly because he wasn't really alone. He'd occasionally warn people not to touch the Pandorica, and it's indicated he's actually working security at the museum where the Pandorica ends up.
The Twilight Zone used this in their first episode, operating from a premise of "if a guy's alone in a spaceship for a few weeks, will he go insane?" Something which, back at the dawn of the Space Age, they genuinely didn't know.
Parodied in Arrested Development. The patriarch of the Bluth family, while in prison, is thrown in isolation for only a few hours. He goes through various forms of insanity as the (short) time passes, and eventually has a religious epiphany.
Tested by the Mythbusters. As the experiment was ended early, it was only determined plausible.
LOST has Rousseau, who lived alone on the island for sixteen years and is now not entirely sane. Desmond also applies to a lesser extent, as he was stuck alone in the hatch for several weeks before the survivors found him and didn't look particularly sane then. Sayid hung a lampshade on Rousseau's madness, saying that she has been alone for too long. And in Desmond's case, it was likely a combination of isolation and not getting enough sleep. Claire is another example.
Clare in the Hyperdrive episode of the same name is a famous spacewoman in a solo trip around the galaxy who has developed, among other issues, paranoia and the delusion that her cup Mr. Cup is talking to her.
The reality TV show Solitary is based on this trope.
In the Stargate SG-1 episode "The Torment of Tantalus," a man is stranded on an alien planet for more than 50 years. When the team stumble across him, he refuses to believe that they're real at first.
Law & Order: SVU: Elliot Stabler spent a few days in solitary (voluntarily) to test a perp's claim that being locked up in solitary for nearly his entire prison sentence (which amounted to over 30 years by the time the perp finally got out) drove him insane and made him more likely to commit violent crimes because he no longer knew how to function in a social environment. Elliot spends a weekend in the same cell, and nearly flips out when he's finally released.
Criminal Intent did this with Goren, showing a day and a half in solitary in about 5 minutes. it was a long five minutes to watch, and when the guy comes to release him Goren practically rips his head off saying "I told you just the weekend!" Cue the guard saying "It's Sunday evening."
Angel overlaps this with With Great Power Comes Great Insanity: The third season episode "Birthday" depicts an Alternate Reality in which Cordelia never reconnected with Angel in Los Angeles and formed Angel Investigations with him, instead becoming a famous actress. Doyle passed his visions on to Angel himself before his Heroic Sacrifice, and Angel retreated into himself in his grief, with the power of the visions not helping matters. Eventually, Angel slipped up to the point where he would have visions of his victims as Angelus, with his only contact with Gunn and Wesley being to inform them of his latest vision and send them out to fix it, with the visions also causing violent episodes that prompted Gunn and Wesley to install chains in his room. When Cordelia comes to see him, Angel is babbling and barely coherent and beats his head off the wall of his room. The worst part of it all is that, according to Wesley, that example of madness is "him on a good day."
On Life, Charlie Crews was a well-adjusted cop and family man...until he was falsely convicted of murder and spent 12 years in prison, the majority of which he spent in solitary confinement. He's not quite all there when he gets out.
The Fades: This is present in the backstory of Big Bad John. A major reason for his descent into madness and villainy was being trapped on Earth as a Fade, unable to interact with anything or anyone, for at least sixty years after he was unable to ascend.
The pilot episode of the original Hawaii Five-0 had enemy agents kidnapping and killing American agents but in such a short time that it seemed impossible that they all talked. McGarrett volunteers to be the next one kidnapped and finds they are using a sensory deprivation tank that very quickly scrambles their brains.
During the three year Time Skip between seasons four and five of Merlin, Morgana and Aithusa were locked at the bottom of a well without sun for two years. Morgana has gone way off the deep end, and Aithusa has gone from a cute, helpful little dragon to an emaciated shell incapable of speech.
In an episode of My Name Is Earl, Earl is trying to repay the government tax money he didn't pay (that they wouldn't accept) by doing some work. He spots a Chain Gang and asks the guard if he can join. The guard agrees, and Earl is happily working on the side of the road. Problem is, while his back was turned, the guards changed shifts, and the new guard had no idea Earl wasn't a prisoner. (It didn't help that Earl was wearing the same kind of jeans and white T-shirt the prisoners were wearing.) When Earl tries to explain that he isn't supposed to be imprisoned, the guard takes that as having an attitude problem and throws him in solitary. As Earl puts it:
Implied to be the fate of the protagonist in Rush's song "Xanadu". His quest for immortality leads him to "the caves of ice", and the second half of the song implies that ever since then he's been trapped there and goes mad from isolation ("the last immortal man") and the prospect of having nothing to do but wait for the world to end.
Deconstructed in Pink Floyd's Concept AlbumThe Wall and its accompanying movie, where the main character Pink experiences a self-inflicted Go Mad from the Isolation after constructing the titular wall around himself, discovering that alienating everyone and everything is much worse than having to deal with them like before.
Pink: There must have been a door somewhere when I came in...
Ben Gunn in Dino Attack RPG has been living in the Goo Caverns for so long he can't even remember who he is. However, he does turn up again near the end a bit saner.
Wallace Bishop's mental health had declined after the lab accident that resulted in the death of his assistant, but it was the years of complete isolation while committed to Napoleon XIV Mental Institution that really drove him into madness.
In Dungeons & Dragons, there was a demonic armor that was cursed so that the wearer could not remove it once equipped. However, the armor was also enchanted to provide sustenance so that the wearer did not have to eat, drink, or sleep. The story goes that one adventurer found and wore the armor. As his party was adventuring through a dungeon, he fell into a pit trap that sealed itself after he fell in. His party didn't notice him fall and never found the trap. The poor adventurer spent decades in the pit going mad before dying of old age.
New World of Darkness has characters who spend incredibly long periods of time alone (say, years) make rolls to avoid degeneration. Failing a degeneration check requires rolling to avoid getting derangements.
While player characters usually needn't worry, the books specify that this is a way to justify insane Storyteller characters (such as, say, a Promethean who's spent so long "going to the wastes" he can't even remember how to talk).
Changeling: The Lost: Changelings have to roll for Clarity if they go a week without human contact; being already traumatized, abused, and delusional, not having others around to reassure one's normalcy and safety is a great way to sink further and further into madness...
To say nothing of Geniuses. They tend toward insanity to begin with, and staying around normal, mundane people with normal, mundane values is one of the main ways they keep themselves anchored and able to understand normal human modes of thinking. For reference, killing people is less bad for a Genius's sanity than going a month without human contact. Going a week without human contact is worse than leaving people to die, or kidnapping. Even going a day without human contact is a sin (but then, so is surgery.)
In Portal the protagonist comes across the makeshift camps of an earlier survivor, Doug Rattmann: isolated for weeks in a death maze presided over by a pathological AI, there is evidence that he had succumbed to insanity, such at the photographs with the faces replaced with pictures of their inanimate Companion Cube. How he managed to even function faced with both schizophrenia and social isolation is a mystery.
In Left 4 Dead 2, in the Last Man On Earth mode, you're the only Survivor (and there's a mysterious absence of Common Infected), but your character still talks as if the others are still there. This is made even creepier by the fact that your character will still periodically shout out something along the lines of "Hello?" as if they're continually looking for the other survivors.
In Mass Effect 1, if you save rescuing Liara for last, you'll find that she has gone half mad from spending so much time in a bubble. She'll refuse to believe that you aren't a hallucination until you physically drag her to safety. She recovers pretty quickly, though.
The Rachni encountered all have gone murderously insane from being separated from their queen.
In the Warcraft III expansion campaign, Maiev accuses Illidan, who spent ten thousand years as a Sealed Evil in a Can cut off from any contact with other intelligent beings, of being insane. He archly replies that yes, isolation will do that to the mind. All the more ironic because she was his chief jailer and, as a result, became completely fixated on him as the only purpose of her life.
In Myst III: Exile, this is partially why the game's antagonist Saavedro wants revenge.
Myst IV explores this trope a couple different ways. Sirius and Achenar were sentenced to two very different forms of solitary confinement. Sirius, the more rational of the two, goes totally nuts because he spent the last twenty years in a place with no indigenous life forms. Achenar, who was unhinged from the start, actually gets rehabilitated from his stretch in a place which has many different forms of life. Conclusion: Isolation is only guaranteed to drive you mad if you are the only sentient life form present.
Implied to have happened with The Twelve Traitors in Lusternia. Granted, they were pretty merciless prior to their exile in the Void, but thousands of years alone wandering the darkness certainly weren't kind to Fain. And as for Morgfyre...
Played straight for Arakune, though. As a human named Lotte Carmine, he continued to isolate himself in his own research to be the scientist supreme for himself, refusing even the only one who wanted to help him, Litchi (the rest could not care less about him at all). When he goes to the Boundary despite Litchi's warning, the corruption got to him easily due to him isolating himself, and thus turning him into Arakune.
Ghost Trick: Yomiel spent years separated from humanity, with his fiancée having committed suicide because he was presumed dead, unable to die, and this is what fueled his need for revenge. Though he wasn't technically alone...
Voldo from the Soul Series lost his sanity as well as his sight from years of being locked in the Money Pit.
This is one of the many, many infernal punishments available in the game of Afterlife, "Screaming Subspace Voids". Lustful Souls are blindfolded, ear-plugged, and trussed up so as to be immobile, then suspended in a pit for a couple hundred years, effectively simulating The Nothing After Death. Insanity Usually Sets in After a Hour.
This almost happened to Laurent in Fire Emblem Awakening. When he and his childhood friends from the bad future decided to attempt Time Travel to save their world, Laurent was accidentally thrown three years before the date they should have arrived to. As a result he spends quite a while on his own in a practically strange land, and up until he finally finds his friends as well as the parent characters, there were times when he was this close to completely lose it.
Invoked in Silent Hill 2 where the atmosphere of the game is designed to "heighten the player's sense of isolation" to contribute to the horror. The first action the player takes is to run down a purposefully long, empty, foggy, narrow path while mysterious footsteps sound behind you with no known source.
In The Elder Scrolls V: Skyrim, Cicero's journal shows that the reason he became the maniacal jester he is now was due to seeing the Dark Brotherhood crumble all around him and being stuck taking care of the Night Mother with a long time with nothing but the memory of his final contract, a jester who simply laughed as he died.
The Nameless Mod: "Welcome to my domain, mortal. I've put up a scrambling field to disrupt all communication in and out of the facility. Let's see how long it'll take before I can make you scream like a little girl." —Shadow Code.
In Borderlands, Patricia Tannis was already a little neurotic when she came to Pandora to research the Eridians, but after being the only member of her team to survive Pandora's various hazards, she started going completely over the edge, doing things like dating (and breaking up with) her ECHO recorder and having extended conversations with the corpse of one of her late team members.
Dawn of War: Turns out that staying in the Warp alone for centuries is, in fact, not good for the mind, as Azariah Kyras can attest.
Though it was a simpler, weaker voice that illuminated me during my centuries upon the Judgement of Carrion. It was Khorne's messenger, who showed me the true path to freedom from our pathetic corpse-Emperor. And what is this path, this meaning, this purpose, to which we gather the skulls of our foes? It is nothing. There is no path. No meaning. We murder. We kill. It is mindless savagery, this universe is mindless.
This is the expected result of the Maximum Obligation in Sharin No Kuni. Although the only time it is shown is an aversion.
Averted in Swan Song, where the businessman that opted against joining a faction isn't only sane, but he's rather helpful and talkative in spite of his situation. He may even be the happiest character in the entire Visual Novel that isn't Aroe, even!
In Umineko no Naku Koro ni, this is stated to be a fate which befalls witches who cause a Logic Error but eventually manage to escape. Including Lambda and Bern, which is the reason they're so messed up now.
Lambdadelta: Hey, are you guys... really real?
In Chopping Block, the serial killer Butch left a woman locked up in his basement with no contact with the outside world to see if it was possible for someone to become bored to death. The story is told from the woman's point of view and ends with her happily telling the room's inanimate objects good bye because Butch has gotten tired of waiting and is just gonna put a power drill through her head.
The point of Ian Samson's strip Idle Minds, where the heroine is disguised as a statue for one week in a big deserted gallery so she can spy on the Big Bad and his sidekick when they visit the place. The isolation, together with her fear that she may have failed in her mission, drives her completely crazy, but she's saved by her subconscious mind.
An example in the comic Penny Arcade did for the Fallout 3 release, featuring a Vault containing one man... and a crate full of puppets. With predictable and insane results. Yes. Yes indeed.
In Drowtales, Diva'ratrika only barely managed to avert this after her daughters betrayed her and collapsed her throne room around her, and she stayed alive because of a single slave that was able to fit through the air ducts and provide some company and sustenance. She managed to stay alive for a year, but in that time her sanity definitely began to slip, and in the end she separates her aura from her body, effectively killing herself, and does a Fusion Dance with Ragini, the slave.
Artificial intelligences in Schlock Mercenary tend to do this...very quickly, since they think at computer speed. This becomes a major plot issue in the "Random Access Memorabilia" story arc, with Tagii.
Subverted by T'kkkuts-Afa. Everyone is terrified of the idea of a super-intelligent AI being trapped with no company for 12 million years, but once contact is actually made, she's just following her original programming. After becoming aware she no longer needs to suppress annie plants, she eventually "retires" and goes on to become the Tough's newest AI.
This is one theory as to why Salad Fingers is so mentally disturbed.
This Grickle short had Santa Claus have this happen to him, leading to a little bit of Fridge Horror that he lives on what amounts to a desolate ice cap. He begins growling like a lion to an audience of elves, and his elves start squealing like monkeys in something that evokes 2001: A Space Odyssey, all while rather creepy music is playing. It's terrifying.
People that get displaced to the realm of SCP-201 for more than three months, as said by the description itself, "suffer lasting psychological damage consistent with being sequestered within solitary confinement".
In The Ren & Stimpy Show episode, "Hermit Ren," the eponymous dog gets so sick of Stimpy he leaves to join a hermit guild. They provide him with a cave and a boulder to lock him in forever. Completely alone. It doesn't take long for him to lose his mind. He gets kicked out for creating imaginary friends.
Likewise, Ren (or Commander Hoek technically) goes insane in "Space Madness" when, confined to a spaceship on a long mission, he is deprived of all contact besides Cadet Stimpy. Interestingly Stimpy does absolutely nothing to instigate this as the only bit of mischief he causes in this episode occurs after Ren is long gone. However, he does say that they've "made this trip dozens of times", so this was something of a slow burn.
Ben 10: Alien Force: Professor Paradox. Originally from The Fifties, he was trying to figure out how to travel through time. Unfortunately, it worked too well and was sucked into the time portal, which then imploded. He spent nearly ten thousand years floating randomly through time, driving him mad. But then he got Bored With Insanity and became "sane...very, very, sane..."
An episode of Batman Beyond has the villain putting patients who act up in "Iso"- isolation units, AKA complete sensory deprivation. At least one of these patients is shown to have sustained permanent psychological damage.
The Penguins of Madagascar: In the episode "All King, No Kingdom", Julien banishes his two followers from their habitat, and soon starts behaving oddly because of having nobody to pay attention to him. He ends up holding a party and inviting his stuffed toys.
SpongeBob SquarePants and Patrick once fell victim to this trope after being trapped in a cave with a crazy old man who convinces them to try to eat each other. Turns out the old man was actually Sandy, and their willingness to resort to cannibalism proved that they were "true survivalists." But Spongebob and Patrick just turn on Sandy and try to eat her.
It happens again in the episode "Gone", where everyone in Bikini Bottom disappears to celebrate National No-Spongebob Day. Spongebob goes from acting out everyone else, to rampaging around town with a boat, to going completely insane by befriending the boat, making a Patrick dummy out of Krabby Patty meat, then rampaging around town to escape from his boat friend.
In My Little Pony: Friendship Is Magic episode "Party of One", Pinkie Pie thinks her friends don't like her anymore and don't want to come to her party, since they have been avoiding her. Being Pinkie Pie, this has an immediate effect on her: as soon as the show returns from the commercial break, we see her holding a party with a collection of inanimate objects, fitting them with party hats and giving them all names and distinct voices.
The Ed, Edd n Eddy episode "Laugh Ed, Laugh" has all the kids in the cul-de-sac, except the Eds, come down with the chicken pox. While Ed and Double-D are able to cope with this, Eddy becomes restless with the lack of kids to scam. Eventually, it becomes too much for Eddy and he snaps from the stress; he spends the rest of the episode scamming squirrels and mistaking fire hydrants for jawbreakers.
Completely averted in Futurama when Bender ends up as just a head buried in the ground for over a thousand years:
Bender: I was enjoying it, until you guys showed up.
Happened in a much briefer way to Brain in Pinky and the Brain. After getting trapped in a sensory deprivation chamber not long after being made intelligent, the isolation made him feral just long enough that the scientists responsible for his intelligence believed their efforts has only produced a mouse that could say "Narf!"
Inverted in Justice League Unlimited. In an alternate timeline, the immortal villain Vandal Savage accidentally killed every other human being on the planet. After 30,000 years with nothing to do but regret his mistake and come to know himself, he went sane.
Wakfu gives us Qilby the Traitor. He wasn't terribly stable in the first place, but being trapped alone in a featureless white void for untold millennia did his mental state no favors.
Orfield Labratories of Minnesota created the quietest isolation chamber in the world for various experiments. NASA uses it for training astronauts to deal with the extreme quiet of space. The effects come on extremely fast, with people starting to have auditory hallucinations and asking to be let out in less than forty-five minutes.
Felicity Aston, first woman to make a solo trek across Antarctica, took almost sixty days. She was not entirely out of contact with the world - she used Twitter the whole way - and "mad" is very much overstating it, but she didn't see anyone else for the whole time. Ashton wrote about “Having to remind myself of the rules now I’m not alone; no peeing wherever I stand, no talking to the sun, no snot or dribble on my face…” and was clearly affected when she finished.
When highly social animals are kept alone in captivity, this can result. Most zoo accreditation organizations require that their members do not house such animals alone.