Snoring is the sound produced by the vibration of soft tissues in the throat as air struggles to pass through a narrowed airway during sleep. This turbulent airflow causes the pharyngeal structures to flutter, generating the characteristic noise. The question of whether snoring occurs, and how loudly, is not consistent across the night; it changes dramatically depending on the specific phase of the sleep cycle. To understand the different patterns of nighttime breathing, it is helpful to examine how the body’s physiology shifts between the two main states of sleep, Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM).
The Mechanics of Sleep Stages
The sleep cycle is broadly divided into NREM and REM stages, each having a distinct effect on muscle tone, which directly influences the upper airway. NREM sleep, which includes the deepest, most restorative stage, is characterized by a general relaxation of most skeletal muscles. However, the muscles responsible for keeping the upper airway open retain some level of protective activity. This residual tone in the pharyngeal dilator muscles works to maintain the airway’s structure against the negative pressure of inhalation, meaning the airway narrows compared to wakefulness but is not maximally collapsible.
As the night progresses, the brain enters REM sleep, a state marked by heightened brain activity resembling wakefulness, coupled with a near-complete loss of muscle tone in the body’s skeletal muscles. This temporary paralysis, known as atonia, affects the muscles of the upper airway, removing the primary defense against collapse. The upper airway dilator muscles become profoundly suppressed, losing the activity present during NREM sleep. This physiological shift significantly increases the collapsibility and resistance of the airway, setting the stage for different types of breathing events.
How Snoring Changes Between Stages
Snoring is generally most frequent and loudest during the deepest stages of NREM sleep. In these stages, the muscles are relaxed just enough to cause a significant narrowing of the pharynx, creating substantial air turbulence. They still possess sufficient tone, however, to allow the soft tissues to vibrate intensely. This state—narrowed but not completely collapsed—is ideal for generating the loud, consistent sound associated with primary snoring.
During REM sleep, however, the nature of breathing disturbances shifts due to the profound muscle atonia. The near-total collapse of the upper airway often prevents the tissues from vibrating, meaning that loud, conventional snoring may decrease or stop entirely. Instead of a loud snore, the complete loss of muscle tone makes the airway maximally prone to full obstruction. This can lead to more severe, but often quieter, breathing events like apneas (cessation of breathing) and hypopneas (shallow breathing).
The breathing events that occur in REM sleep can be more damaging because the body’s response to low oxygen is less effective, and the respiratory rhythm is more irregular than in NREM sleep. While simple snoring is less common in REM, the breathing disturbances associated with Obstructive Sleep Apnea (OSA) are often at their most severe in this stage. The complete lack of protective muscle tone means the airway is most unstable, resulting in a higher rate of severe respiratory events.
Diagnostic Importance of Stage-Dependent Snoring
The distinct patterns of breathing in NREM versus REM sleep are important for diagnosing and treating sleep-related breathing disorders. Sleep specialists use polysomnography, a comprehensive sleep study, to measure the frequency of apneas and hypopneas in both NREM and REM states. This allows for the calculation of stage-specific Apnea-Hypopnea Index (AHI) values, which can vary widely.
Many patients with OSA experience a significant worsening of their condition during REM sleep, a phenomenon known as “REM-predominant OSA.” This diagnosis is made when the AHI is substantially higher during REM sleep than during NREM sleep. Recognizing this distinction influences the overall clinical picture and the choice of treatment.
For example, if a patient’s breathing disturbances are primarily limited to NREM sleep or occur only when sleeping on their back, strategies like positional therapy might be an effective option. Conversely, REM-predominant OSA indicates a breathing issue that is less dependent on physical position and more on the physiological state of atonia. This often requires more continuous support, such as a Continuous Positive Airway Pressure (CPAP) machine. Understanding the specific sleep stage where the airway instability is greatest is fundamental to creating a targeted and effective treatment plan.