Blowing raspberries, a fluttering, vibratory sound made with the lips during sleep, is a specific and often startling nighttime behavior. While it sounds like a deliberate, playful action, the noise is entirely involuntary, typically occurring when the sleeper is unaware. This vocalization is a specialized sleep sound. Understanding its cause requires looking at the physiological changes that occur as the body rests, ranging from simple mechanical airflow to temporary neurological activity.
The Biology Behind Sleep Sounds
The ability to produce any sound during sleep is closely tied to the relaxation of muscles in the upper airway and throat. As the brain cycles through different sleep stages, muscle atonia sets in, which is a temporary paralysis of most voluntary muscles. This atonia is most complete during rapid eye movement (REM) sleep and is a protective mechanism that stops us from acting out our dreams.
The muscles controlling the airway, including the tongue and the soft palate, relax significantly during all sleep stages. This relaxation decreases the muscle tone that normally keeps the throat open during wakefulness. The primary upper airway dilator muscle, the genioglossus in the tongue, has its activity reduced as sleep deepens.
When air is inhaled or exhaled past these relaxed tissues, the resulting turbulence causes them to vibrate, generating sounds like snoring or groaning. The raspberry sound involves the lips, which are also voluntary muscles whose tone decreases during sleep. This specific noise is created when air is forced out through the lips while they are in a state of partial, floppy relaxation, causing the lips to flutter against each other.
Common Explanations for Nocturnal Raspberries
The distinct raspberry sound can be classified into two primary mechanisms: neurological and mechanical. In the neurological category, the sound is often a form of parasomnia, or an undesirable physical event occurring during sleep. This may be a brief, isolated vocalization, similar to sleep talking or a sudden grunt, often happening during transitions between non-REM sleep stages when motor function momentarily activates.
This vocalization can also be a benign developmental stage in infants and toddlers, who are actively practicing and exploring new motor skills. These newly acquired movements sometimes spill over into their lighter sleep cycles as the brain processes and consolidates the motor pattern. For adults, the sound is rarely related to motor learning, but instead can be a specific kind of sleep-related vocalization, where the brainstem briefly signals the mouth muscles.
The mechanical cause involves air pressure, which is particularly relevant for individuals using a continuous positive airway pressure (CPAP) machine for sleep apnea. The pressurized air from the device can sometimes leak from the mouth, vibrating the relaxed lips to create a sound identical to a raspberry. Even without a device, if the mouth opens slightly due to muscle relaxation, a strong exhale can cause the lips to flutter, producing the characteristic noise due to air turbulence.
Related Conditions and When to Seek Medical Advice
While most instances of blowing raspberries during sleep are harmless isolated events, the behavior can occasionally be linked to broader sleep disorders. In children, especially, repetitive movements or noises can be part of Rhythmic Movement Disorder (RMD). This disorder involves repeated motions like head banging or body rocking, sometimes accompanied by rhythmic humming or other sounds.
While RMD is common in infancy and usually resolves, if the noises are highly repetitive, disruptive, or continue into later childhood, it warrants a medical evaluation. For adults, the noise might be confused with the gasping, snorting, or choking sounds that characterize Obstructive Sleep Apnea (OSA). OSA occurs when the airway fully or partially collapses, and the resulting noises are symptoms of breathing distress, not a simple lip vibration.
If the raspberry sound accompanies significant daytime fatigue, pauses in breathing, or a feeling of waking up gasping for air, a sleep specialist should be consulted. A consultation is also advised if the nocturnal vocalizations are frequent, disruptive, or accompanied by complex body movements. This may suggest a form of parasomnia like REM Sleep Behavior Disorder (RBD).
In RBD, the normal muscle paralysis during dreaming is absent, allowing a person to act out their dreams with vocalizations and movements. Seeking medical advice is generally recommended if any unusual sleep behavior consistently disrupts sleep quality or causes concern for the sleeper or a bed partner.