The relentless dangers of workplace noise—a hidden root cause of sleep disorders

Noise is an unavoidable hazard in modern society. Whether it's the endless stream of cars in the city, the roar of airplanes taking off and landing, the non-stop activities of factories day and night, or even neighbors arguing, children crying, and pets barking, noise can not only cause hearing loss, but it can also disturb people's sleep.

Sleep is the primary mechanism for bodily repair in humans. Adequate sleep maintains normal brain activity, physiological behavior, and mood stability. According to the 2007 manual of the American Academy of Sleep Medicine (AASM), sleep consists of four cycles. The typical sleep cycle for adults, as shown in Figure 1, includes three stages of non-rapid eye movement (NREM) sleep (N1, N2, N3) and rapid eye movement (REM) sleep, with approximately 4-5 cycles occurring each night. The initial stage of sleep is NREM stage 1 (N1), during which the eyes move slowly and the body relaxes. In stage 2 (N2), the heart rate gradually slows and body temperature gradually decreases. Stage 3 (N3) is deep sleep, where muscles are completely relaxed, and heart rate, respiratory rate, and body temperature are at their lowest. At this stage, it is difficult to be awakened, and even if forcibly awakened, it takes time to regain consciousness. NREM usually occurs in the pre-sleep phase, during which the body secretes growth hormone and other substances that promote protein synthesis and repair damaged tissues. During REM sleep, the eyes move from side to side and the brain is active. This stage is believed to help digest and incorporate the experiences before sleep into memory, which is helpful for memory and learning.

After a day of mental exertion, brain cells and tissues accumulate stress and damage. Repair during the NREM (Near-Nearest Sleep) phase is crucial for maintaining life; prolonged sleep disturbances can severely harm health. Factors contributing to sleep disorders include age, psychological factors (anxiety, tension), illness, medication, and environment. Noise is a common factor contributing to poor sleep environments. Studies have confirmed that noise disturbances reduce the proportion of deep sleep (especially N3), make people more prone to waking up, and even impair the body's self-repair mechanisms. Regarding noise in the sleep environment, the World Health Organization (WHO) guidelines in 1999 recommend that the average continuous sound pressure level in a sleep environment be below 30 dB, and the instantaneous maximum sound pressure level not exceed 45 dB. However, in urban areas, this standard is often difficult to achieve. Therefore, attention should be paid to the soundproofing of the bedroom (reducing noise by about 15 dB compared to the outdoors) and reducing the noise levels of air conditioning and ventilation equipment to minimize environmental noise interference with sleep.

Besides the effects of environmental noise, prolonged exposure to high noise levels during the day can easily excite the sympathetic nervous system, leading to a faster heart rate, higher blood pressure, and a continuous secretion of cortisol, a stress hormone. Studies have shown that excessive cortisol accumulation during the day can extend feelings of excitement into the night, similar to continued exposure to noise at night. Furthermore, elevated cortisol levels can decrease sleep quality, increase nighttime awakenings, and result in a feeling of insufficient recovery after sleep (representing a feeling of not getting enough sleep), clearly highlighting the impact of cortisol on individual sleep quality and the body's repair mechanisms.

To avoid the endocrine effects caused by noise in the work environment during the day, in addition to employers controlling noise sources in the workplace, workers should cooperate with employers and occupational safety and health personnel in implementing hearing protection measures, such as following engineering improvement suggestions, reducing noise exposure during rest periods, and wearing personal protective equipment. Reducing noise exposure during the day can not only prevent hearing damage caused by noise, but also reduce excessive cortisol secretion, allowing the body to rest fully and maintain health.

Source: Labor and Occupational Safety and Health Briefing No. 4 / Occupational Health Research Group, Huang Yuping