Master of Your Domain

"Paul sensed his own tensions, decided to practice one of the mind-body lessons his mother . Three quick breaths triggered the responses: he fell into the floating awareness ... focusing the consciousness ... aortal dilation ... avoiding the unfocused mechanism of consciousness ... to be conscious by choice ... blood enriched and swift-flooding the overload regions ..."

- Dune

The human body is an amazing machine, the brain even more so. It is also a frustrating one at times, often acting as if it has ... well, a mind of its own. Many bodily functions are handled without our direct, conscious control, such as pulse and respiration rate, digestion, and even how we register pain. There are those who have sought to challenge the authority of the autonomic via Enlightenment Superpowers: ascetics and warriors, yogis and sadhu, Tibetan monks and Zen masters. Western science took the ball and ran with it to extract the concept of "biofeedback", a method of consciously controlling normally automatic functions of the body. The real thing is extraordinary enough (see Real Life below) but, predictably, fictional works often exaggerate it to the point of Charles Atlas Superpower.

For the more extreme cases, it's often Hand Waved by 90% of Your Brain and other cases of Art Major Biology. The title is decidedly not a Seinfeld reference.

Anime & Manga

 * Gundam Wing has Heero Yuy, who could control his brainwave activity.
 * The Big Bad from Ninja Scroll mastered this to a deeply exaggerated level; he can control every single cell of his body and regenerate from any level of damage short of total annihilation.
 * Medaka of Medaka Box is able to consciously control her reflexes and sundry other biological processes.
 * Rob Lucci and Kumadori of the One Piece villains CP9 have this ability which is called Life Return (Seimei Kikan). Kumadori is the one who uses it most, allowing him to do things such as control his long hair as a weapon or instantly digest a large amount of food. Lucci on the other hand can use it in conjunction with his Zoan Devil Fruit powers to make his large Leopard-Man form more proportionate to his human form.
 * Chopper has gained this to an extent as post Time Skip he no longer needs the Rumble Ball for six out of his seven forms, when previously he needed it for any transforming beyond the three-form Zoan standard.

Literature

 * The Bene Gesserit of Dune have such utter mastery of their bodies that they could all be immortal if they wanted to. They choose to age only because a cult of eternally-young women would be socially destabilizing.
 * The Dunyain from the Prince of Nothing books by R. Scott Bakker are essentially unisex Bene Gesserit with a few Mentat skills for good measure.
 * The Envoys from Richard K. Morgan's Altered Carbon (and sequels) are all capable of manipulating their bodily functions. In Broken Angels, Takeshi Kovacs
 * Cory Doctorow's story "0wnz0r3d" features computer hackers that figured out how to hack their own bodies. Among other things, they spawn custom T-cells that infect HIV-positve people and cure them.
 * Almost every book by Poul Anderson features biofeedback. The Boat of a Million Years has Hanno the immortal who learned to accelerate his own already enhanced healing (he was an immortal after all).
 * The Monastics from The Acts of Caine books have their Control Disciplines.
 * The Spine Assassins of the Deepgate Codex books are "Tempered" into emotionless killing machines who can suppress pain responses and accelerate their metabolisms for brief bursts of super speed.
 * Used in the Star Wars Expanded Universe. From putting yourself into a trance that allows you to seem dead, to Vergere's 'Art of the Small' which she used to alter her own bodily chemistry until her tears could inflict or cure diseases if she wished, it's all over the place.
 * In The Symphony Of Ages books by Elizabeth Hayden, Achmed the Snake can change his heartbeat to match the rhythm of anyone possessed by a demon or from the island of Serendair in order to track them across long distances.
 * Anne McCaffrey's Dinosaur Planet books feature "Discipline": a full-featured body-control/pain-control/emotion-control/adrenal-control technique that many of the characters practice.
 * Ursula K. Le Guin's The Left Hand of Darkness has the dothé discipline which enables practitioners to deliberately unleash hysterical strength
 * In Jasper Fforde's Shades of Grey: the Road to High Saffron, can consciously control, which means that unlike everyone else in the novel's dystopian setting,.
 * The Red Monks from Tim Lebbon's Dusk and Dawn have this talent alongside focused berserker rage/hate.
 * The protagonist from Ben Bova's Orion has complete awareness and control of every synapse in his nervous system.
 * Justin Gilead from obscure 80s spies-chess-and-Shangri La novel Grandmaster by Warren Murphy and Molly Cochran had this in addition to a larger suite of Charles Atlas Superpowers.
 * This appears in many of Vonda N McIntyre's books, including both Dreamsnake and the Starfarers quartet. In each of them, some biofeedback ability, including the ability to render oneself temporarily sterile by altering the temperature of one's genitalia, is standard for most adults.
 * Harry Dresden learns a few techniques from his interactions with Lash: mostly pain suppression but also uses yogic contortionist techniques to escape the thorn manacles in Proven Guilty.
 * The protagonist of the Iron Druid Chronicles is able to draw energy from the earth to heal injuries, purge toxins and negate pain.
 * Remo Williams of The Destroyer uses the principles of Sinanju and its total body awareness to do everything from regulate his body temperature to expel poisons from his body.
 * Danica of The Cleric Quintet uses this among her various monk abilities. At one point, she consciously reverse blood flow in her body to force poison out.
 * The character of Emma Anyanwu and her descendants in Octavia Butler's Patternist series can control their bodies at a cellular level. Anyanwu can alter her own DNA and appearance at will, while her descendants are able to heal or injure the bodies of themselves and others. However, it takes time and practice to learn to do this properly, and every change they make in a body has to be understood and visualized.
 * The protagonist of Ayize Jama-Barrett's The Liminal People can do nearly anything to the cells of his own body or others.

Live Action TV

 * The Bionic Woman (1970s) episode "Biofeedback", which had a guest character who could use biofeedback to control his own bodily functions.
 * In the Star Trek episode "Where No Man Has Gone Before," the newly psychic Gary Mitchell can control his heart rate.
 * In one episode of The Incredible Hulk (1970s TV show), the main guest character was a cop who was also involved in martial arts, who could turn his pulse off during meditation. He and his sensei tried to help David to use meditation to suppress the change.
 * In The Invisible Man series pilot, Darien is having trouble consciously controlling his Quicksilver gland, which is activated by adrenaline. This is hilariously shown when he tries to spy on a soldier and a nurse getting it on and reappears at an inopportune moment (which the soldier did not appreciate). His brother teaches him some basic yoga techniques to allow him to not only appear and disappear at will but to also selectively make parts of his body invisible. He also learns to secrete Quicksilver on small objects, making them temporarily invisible as well.
 * This is brought up in a later episode where his brother's RNA is injected into the gland, allowing him to temporarily take control of Darien's body (don't ask). He also has trouble controlling the gland at first.
 * Stanton Parrish from Alphas has this to the point that he hasn't aged since the American Civil War.

Comic Books

 * Batman has done this tons of times, usually in the form of slowing his heart to fake death or conserve oxygen.
 * Superman has done it too, slowing down his heartbeat to fake death and escape imprisonment as Clark Kent.
 * Issue #10 of Warren Ellis' Global Frequency is pretty much a 20-page fight between two guys who can do this. It is predictably messy.
 * In Stormwatch: Team Achilles (#23 or so), Baron Chaos fools The Authority's appallingly high-tech scanners into thinking he's in a coma.
 * Daredevil, thanks to his training with Stick, can speed up his healing via meditation. It's a safe assumption that Elektra can too. Probably Shang Chi as well.
 * Noh-Varr, of the Grant Morrison comic Marvel Boy, has a technique for rerouting pain signals to his auditory cortex i.e. turning pain into music. There is also the White Run technique from issue #4.
 * An Old Master during the "Forever" story arc in Powers could do this.

Film

 * Raizo from Ninja Assassin survives hideous injuries by using this technique. So does the Big Bad. So do the mooks.
 * The ability to consciously slow down one's own heartbeat to fake death is used to escape imprisonment by James Bond in Die Another Day.
 * Our Man Flint: Derek Flint could use meditation to put himself into a state of suspended animation that was indistinguishable from death (no pulse or breathing).
 * The 2008 The Incredible Hulk movie has Banner learning to suppress his transformations with ujjayi breathing and meditation..
 * Dieter Tautz, a yoga master in the film Scanners is said to be capable of controlling his heart rate and several other usually uncontrollable body functions.

Western Animation

 * In one episode of the Justice League, the Flash feigns death by making his heart beat so fast that it looks like he's flat-lining.

Tabletop RPG

 * Metabolism Control in GURPS allows the character to control all autonomic biological functions.
 * There's also the older skill Breath Control, which is classed as Mental/Very Hard and is "not normally taught outside Oriental cultures." It triples the time you can spend underwater, among other benefits.
 * Dungeons and Dragons had the Suspend Animation psionic ability that allowed the practitioner to feign death.
 * Almost entire Psychometabolism discipline consists of things from bursts of strength to rapid healing to sweating acid.
 * Paranoia: the mutant abilities of Adrenalin Control (increase Strength and Endurance), Chameleon (skin pigments) and Suspended Animation.
 * Traveller Classic. The psionic ability of Awareness allowed Suspended Animation, the Regeneration of injuries and enhanced Strength and Endurance.
 * Pops up a few times in New World of Darkness as a low-level power in physical-based power sets, such as the Life Arcanum in Mage: The Awakening or the Corporeum Transmutation in Promethean: The Created.
 * The Star Wars d20 RPG allows trained Jedi to enter a hibernation trance that drastically reduces their need for air, water and food.

Web Original

 * The ASH Universe has Aaron Zander aka Contact, a superhero whose power set includes a physics-violating version of this.
 * It also has a Daredevil-inspired vigilante called Warden whose power is essentially "mind-over-everyone's-body" -- including his own.

Real Life

 * "Biofeedback" is a method of body function control through the use of biometric equipment for, well, feedback i.e. compensating for limited self-awareness with electronics and learning to handle whatever it measures on your own.
 * Thích Quảng Đức, a Buddhist monk in Vietnam, set himself on fire as a means of protest against the policies of the Diem regime. He then sat down on the road and quietly burned to death without screaming in pain or otherwise reacting in any way to the fact that he had just set himself on fucking fire!
 * Auto-hypnosis is another technique for autonomic self-control. Also there is the trick of "thinking your hands warm" to deal with migraines.
 * In one recorded incident involving hypnosis, the subject relived a heart attack. Their heart stopped for 22 seconds (they were hooked up to a EKG). Sometimes the line between fiction and reality is farther out than we think.
 * David Berglas, Derren Brown and many other magicians perform an effect where an audience member, preferably medically qualified, monitors their pulse, tapping in time with it on a cymbal or similar for the benefit of the rest of the audience. Once a clear rhythm is established, the performer makes their pulse slow down, become erratic, and eventually stop altogether.
 * This is a classic illusion in the world of mentalism, but isn't actually an example of this trope since the performer only pretends to use mental powers to slow and stop his or her pulse. Notice how the performer always instructs the volunteer to take the pulse in the wrist, rather than the brachial or carotid.
 * Some women are said to be capable of 'thinking-off'. In other words, will herself to orgasm without any physical stimulation.