People adapt to living near a road or under a flight path and report that they no longer hear it. The physiological response to that sound does not adapt in the same way, which is why noise exposure is treated as an environmental health question rather than a matter of preference.
Hearing has no off switch
Vision can be closed off by shutting the eyes. The auditory system has no equivalent, because it evolved as a threat-detection channel that had to keep working while an animal slept.
Sound is processed in the brainstem before it reaches the parts of the brain responsible for conscious awareness. A loud or sudden noise triggers a response before any decision about whether it matters.
Habituation happens at the level of attention. The signal still arrives and is still processed; what changes is whether it reaches awareness and gets reported.
The stress response is triggered below awareness
Unexpected sound produces a brief cascade including a rise in heart rate, a small increase in blood pressure and the release of stress hormones.
Each individual response is minor and settles within minutes. The concern in noise research is repetition, since an environment that produces the response many times a day never allows a full return to baseline.
Because the person is not consciously reacting, there is no behavioural sign that anything is happening. Self-reported annoyance and measured physiological response are only loosely related.
Sleep is where the effect concentrates
During sleep, the auditory system continues to monitor the environment. Sound can shift a sleeper into a lighter stage or produce a brief arousal without waking them enough to remember it.
These fragmentations do not register the next morning as a disturbed night, but the architecture of the night has changed, with less time spent in the deeper stages.
This is the mechanism behind the association between night-time transport noise and daytime tiredness among people who insist the noise does not bother them.
Intermittent sound matters more than steady sound
Continuous noise at a constant level is easier for the auditory system to filter than sound that varies. Variation carries information, and the system is built to detect change.
A single passing vehicle at intervals through the night can therefore be more disruptive than a steady hum at a higher average level.
Noise regulation that relies on averaged measurements over a period can understate this, which is why some standards include separate limits for peak events.
Why quiet is treated as a health resource
Reducing exposure works through distance, insulation and timing rather than through tolerance, since tolerance operates on perception rather than on the underlying response.
Environmental agencies in many countries map noise exposure alongside air quality for this reason, treating both as involuntary exposures rather than lifestyle choices.
Persistent sleep disruption from any source, including noise, is worth raising with a clinician rather than accepted as an unavoidable feature of where someone lives.