A hiccup, medically known as singultus, is an involuntary, spasmodic contraction of the diaphragm, the dome-shaped muscle located beneath the lungs. This sudden action causes a rapid intake of air that is abruptly halted by the closure of the vocal cords, producing the characteristic “hic” sound. Since the action is entirely involuntary, many people wonder if sleep, which diminishes conscious control, prevents this reflex from occurring.
The Anatomy of a Hiccup
The hiccup mechanism is driven by a reflex arc involving specific nerve pathways and a central processing area in the brainstem. This reflex begins with the afferent, or sensory, limb, primarily composed of the vagus and phrenic nerves. These nerves transmit irritation signals from the stomach, esophagus, or other areas toward the central nervous system. The brainstem acts as the involuntary command center for the reflex.
The efferent, or motor, limb carries the resulting command outward to the muscles involved in the spasm. This pathway relies heavily on the phrenic nerve, the sole motor supply to the diaphragm, causing the muscle to contract sharply. Simultaneously, the recurrent laryngeal nerve causes the glottis—the opening between the vocal cords—to snap shut about 35 milliseconds later. This rapid closure against the suddenly inhaled air creates the distinct sound of a hiccup.
Can the Body Hiccup While Asleep?
The body can indeed hiccup while asleep because the underlying mechanism is a brainstem reflex that does not require conscious input or wakefulness. This reflex is an ancient, hard-wired response, unlike voluntary movements that are suppressed during sleep. The involuntary nature of the reflex arc means the pathway remains active and capable of firing even when the conscious brain is dormant.
Research using polysomnography has confirmed the presence of hiccups across all phases of the sleep cycle. However, the frequency of the reflex firing is influenced by the depth of sleep. Hiccups are most frequent during wakefulness and light sleep stages.
As the brain moves into deeper slow-wave sleep (SWS) and rapid eye movement (REM) sleep, the incidence of hiccups tends to decrease. This suggests that the overall neural activity associated with the hiccup central connection is modulated by the sleep state. If a strong enough irritant is present, the reflex can still penetrate even the deepest stages of sleep.
Triggers for Nighttime Hiccups
The triggers for hiccups at night often involve physical factors that irritate the diaphragm or the involved nerve pathways while a person is lying down. Gastroesophageal Reflux Disease (GERD) is a frequent cause of nocturnal hiccups, as stomach acid flows back into the esophagus more easily horizontally. This backflow irritates the tissues surrounding the phrenic and vagus nerves, initiating the hiccup reflex.
Eating a large meal or drinking carbonated beverages just before bedtime can distend the stomach, putting direct pressure on the diaphragm muscle. This physical pressure acts as a mechanical irritant, transmitted through the phrenic nerve to trigger the spasm. Additionally, digesting a large meal causes the stomach to produce more acid, increasing the risk of reflux-related nerve irritation.
To reduce the likelihood of these spasms occurring at night, lifestyle adjustments can be beneficial. Avoiding meals and alcohol for two to three hours before lying down minimizes stomach distension and acid production. Elevating the head of the bed uses gravity to prevent stomach contents from refluxing into the esophagus, reducing irritation to the reflex nerves.