Hiccups don’t seem to serve any obvious purpose in adults, and that’s exactly why scientists have spent decades trying to figure out why the reflex exists at all. The short answer: hiccups are most likely a leftover from infancy, when they helped nursing mammals expel swallowed air from their stomachs to make room for more milk. In grown humans, they’re essentially a reflex without a job, firing in response to triggers that irritate the nerves controlling the diaphragm.
What Happens in Your Body During a Hiccup
A hiccup is a reflex, not a voluntary action. It starts with a sudden, involuntary spasm of the diaphragm, the dome-shaped muscle beneath your lungs that drives breathing. That spasm yanks air into your chest, and about 35 milliseconds later, the flap at the top of your windpipe (the glottis) slams shut. Air gets violently cut off mid-breath, and that abrupt closure is what produces the “hic” sound.
The reflex travels along a specific nerve loop. Signals come in through the phrenic nerve, the vagus nerve, or nerve fibers running along the upper spine. Those signals reach the brainstem, which fires a response back out, primarily through the phrenic nerve, to trigger the diaphragm contraction and glottis closure in rapid sequence. There’s no single “hiccup center” in the brain. Instead, the reflex uses a loose network of connections in the brainstem, which is part of why hiccups can be so hard to stop once they start.
The Leading Theory: A Burping Reflex for Infants
The most compelling explanation comes from a 2012 hypothesis published in the journal BioEssays. Rather than treating hiccups as a meaningless glitch, researcher Daniel Howes proposed that hiccups evolved as a surprisingly sophisticated reflex to remove air from the stomachs of young suckling mammals.
Here’s how it would work. When a nursing infant swallows air along with milk (which is nearly inevitable during continuous suckling), that air bubble sits in the stomach just below the diaphragm. The presence of that air triggers the hiccup reflex. The diaphragm contracts and the chest expands, but because the windpipe snaps shut at the same time, no air actually enters the lungs. Instead, the sudden drop in chest pressure pulls the air bubble upward from the stomach into the esophagus. Once the hiccup relaxes, the air travels up and out of the mouth. It’s essentially a built-in burp.
This theory neatly explains several things that other explanations can’t. Hiccups are far more common in infants than in adults. Newborns can spend up to 2.5% of their day hiccupping. A reflex that clears swallowed air would give nursing mammals a real survival advantage by freeing up stomach space for more milk during each feeding. As mammals grow and stop nursing, the reflex becomes less useful but never fully disappears.
Why Adults Still Get Them
If hiccups are mainly useful for nursing infants, why do adults get them at all? Because evolution doesn’t remove a reflex just because it stops being helpful. It simply becomes less frequent. The nerve pathways that once served a purpose remain wired into the brainstem, and various triggers can still set them off.
The most common triggers for ordinary hiccups include:
- Eating too much or eating too quickly, which distends the stomach
- Drinking carbonated beverages, which fill the stomach with gas
- Drinking too much alcohol
- Sudden temperature changes, like drinking something very hot or cold
- Swallowing air while chewing gum or smoking
- Emotional excitement or stress
Notice a pattern: many of these triggers involve stomach distension or irritation of the diaphragm and vagus nerve. That fits perfectly with the idea that the reflex originally responded to air in the stomach. In adults, the same nerve pathways get activated, but the hiccup doesn’t accomplish anything useful. It’s a reflex firing out of context.
The Older Theory: A Leftover From Amphibians
Before the suckling hypothesis gained traction, the dominant idea was that hiccups are a vestigial remnant of gill breathing in our amphibian ancestors. Tadpoles use a motor pattern strikingly similar to hiccups when they pump water over their gills: a sharp intake followed by closure of the airway. The argument was that this ancient respiratory pattern got preserved in the mammalian brainstem long after lungs replaced gills.
The amphibian theory explains why the reflex exists in the nervous system, but it doesn’t explain why hiccups would still be so common, especially in infants. A reflex that lost its function hundreds of millions of years ago would be expected to fade into extreme rarity. The suckling hypothesis picks up where the amphibian theory leaves off, suggesting the reflex was repurposed rather than simply left over.
When Hiccups Signal Something Else
Most hiccup bouts last minutes to hours and resolve on their own. These are classified as acute hiccups and are harmless. Hiccups lasting more than 48 hours are considered persistent, and those lasting longer than a month are classified as intractable. Both are relatively uncommon but can seriously affect quality of life, disrupting sleep, eating, and concentration.
Persistent or intractable hiccups sometimes point to an underlying condition irritating the nerves involved in the reflex. Problems in the central nervous system, metabolic imbalances, or issues in the chest or abdomen can all keep the hiccup loop firing. If your hiccups haven’t stopped after two days, that’s worth getting checked out.
What Actually Works to Stop Them
The classic home remedies, breath-holding, drinking water upside down, getting startled, breathing into a paper bag, are all widely used but have never been rigorously tested in clinical studies. That said, there’s a plausible mechanism behind several of them. Breath-holding and paper-bag breathing both raise carbon dioxide levels in the blood, a state called hypercapnia, which can interrupt the hiccup reflex arc. A sudden fright may do the same thing by briefly altering your breathing pattern.
One technique with promising early results is called active prolonged inspiration. You breathe in as deeply as you can, then continue trying to inhale with an open throat for a total of 30 seconds, followed by a slow exhale. In a small clinical study, all 21 patients who tried this method reported immediate relief. The technique works by simultaneously stimulating the phrenic and vagus nerves while building up carbon dioxide, essentially hitting the hiccup reflex from multiple angles at once.
For intractable cases that don’t respond to physical maneuvers or medication, more invasive options exist, including nerve blocks targeting the phrenic nerve (performed with ultrasound guidance) and vagus nerve stimulation. Neither approach works universally, and good data on long-term cure rates is still limited.