The cough reflex is a protective mechanism designed to forcefully clear the airways of irritants and foreign particles. During the day, minor irritations—such as dust or excess mucus—frequently trigger this reflex, keeping the lungs safe. Once we fall asleep, however, this response nearly disappears, suggesting the brain actively modifies this defense system at night. This physiological alteration involves a widespread shift in nervous system activity, which raises the threshold for triggering a cough.
How the Cough Reflex Works
Coughing begins with the activation of specialized sensory nerve endings, known as cough receptors, found throughout the respiratory tract. These receptors are concentrated in the larynx, trachea, and the carina, the point where the windpipe divides into the main bronchi. When an irritant stimulates these receptors, an electrical signal travels along the afferent pathway via the vagus nerve (Cranial Nerve X).
This impulse travels directly to the brainstem, reaching the central coordinating region called the cough center, located within the medulla oblongata and pons. The center integrates the sensory input and determines if the stimulus warrants a response. If the threshold is met, the center generates an efferent signal, which is the motor command to initiate a cough.
The efferent pathway sends signals to the diaphragm, abdominal muscles, and laryngeal muscles. This triggers three phases: deep inspiration, a compressive phase where the vocal cords snap shut, and a rapid expiratory phase. The final phase expels air at high velocity to dislodge the irritant.
General Central Nervous System Changes During Sleep
Sleep fundamentally transforms how the central nervous system processes stimuli. As the brain cycles through sleep stages, it prioritizes restorative functions and works to maintain continuous sleep. This involves the general attenuation, or “gating,” of sensory information that could otherwise lead to arousal.
The threshold for responding to sensory input is significantly elevated during sleep. Minor disturbances are often ignored by the sleeping brain. The motor system also changes, notably during Rapid Eye Movement (REM) sleep, where muscle paralysis, or atonia, sets in.
This reduction in sensory processing and motor output establishes a quiescent environment protecting sleep. The brain filters out low-level irritants that might trigger a cough during wakefulness, setting the stage for specific suppression.
The Specific Suppression of the Cough Center
The lack of nocturnal coughing is due to the targeted inhibition of the brainstem’s cough center. During sleep, the central nervous system raises the activation threshold of this reflex circuit. An irritant must be substantially more intense than a daytime trigger to generate a response.
This dampening is mediated by a shift in neurotransmitter activity. The release of inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), increases during sleep. This increased GABAergic activity suppresses the excitability of neurons in the medulla that are part of the cough reflex loop.
Simultaneously, arousal-promoting neurotransmitters, including serotonin and norepinephrine, decrease. Their reduced presence further deactivates the cough center, making it less sensitive to incoming signals from the vagus nerve.
The cough center’s threshold is highest during REM sleep. Suppressing this reflex ensures the protective mechanism does not interrupt the restorative function of sleep.
Causes of Coughing That Break Through Sleep
Sleep’s inhibitory mechanisms can be overwhelmed by strong stimuli or underlying health conditions. When a person is woken by a cough, the irritant has exceeded the significantly raised central threshold, often due to chronic issues exacerbated by the recumbent position.
Gastroesophageal Reflux Disease (GERD) is a frequent culprit, as lying flat allows stomach acid to flow back into the esophagus and upper airway. This chemical irritation is a powerful stimulus that overrides the brainstem’s nocturnal defenses.
Severe Postnasal Drip causes mucus to pool in the throat, providing an intense mechanical trigger. Conditions like asthma or chronic obstructive pulmonary disease (COPD) also lead to nocturnal coughing. Airway resistance naturally increases during sleep, generating a strong signal that breaks through the central suppression.