Noise is classified alongside air quality as an environmental exposure in public health frameworks. The reason is that the body's response to sound is not limited to hearing it.
The auditory system does not switch off
Hearing evolved partly as a warning system, and it remains active during sleep because a threat could arrive at any hour.
Sound reaching the ear is processed by structures that connect to arousal and stress pathways before, and independently of, conscious perception.
A sleeping person can therefore respond physiologically to a sound they will not remember, which is what separates noise from most other nuisances.
The response is a stress reaction
Sudden or unpredictable sound triggers activation of the sympathetic nervous system: heart rate rises briefly, blood vessels constrict, stress hormone release increases.
These reactions are appropriate and short-lived in response to a genuine event.
Repeated activation, many times a night over years, is the pathway through which chronic exposure is thought to affect cardiovascular measures.
Sleep fragmentation is the main mechanism at night
Noise during sleep causes brief arousals and shifts from deeper to lighter stages, without necessarily waking the person.
Sleep quality can therefore be degraded while total sleep duration looks unchanged, which is why self-report understates the effect.
Adaptation is partial: people report becoming used to noise, while measured physiological responses to it persist.
Character matters as much as level
Measured loudness alone predicts annoyance and disturbance poorly. Intermittent sound disturbs more than steady sound at the same level.
Low-frequency components travel further and penetrate walls more readily, which is why distant sound is often perceived as more intrusive than its measured level suggests.
Perceived control over the source also affects the response, so identical sound is experienced differently depending on its origin.
Exposure is unevenly distributed
Noise from roads, railways and flight paths is concentrated in particular areas, and housing in those areas is typically cheaper.
Exposure therefore tracks other markers of disadvantage, which is one reason it is treated as an environmental health issue rather than an individual one.
It is also largely outside personal control, since the source sits beyond the boundary of the home. Mitigation happens through building standards, glazing and planning decisions rather than through behaviour.
Occupational and recreational noise are a separate matter, since those involve levels high enough to damage hearing directly rather than acting through the stress pathway.
Persistent ringing in the ears, or hearing that seems reduced after exposure, are reasons to have hearing assessed rather than to wait for it to settle.