Metabolic

Noise Effects on Sleep and Health Consequences : The Hearing Journal

Noise is defined as unwanted and/or harmful sound.1 While the general population and certainly most readers of this journal are well aware that exposure to high sound pressure levels contributes to hearing loss and tinnitus, noise affects our health in many other ways beyond hearing.2 For example, numerous studies have shown that noise exposure is associated with community annoyance,3 disturbance of communication, and reduced academic performance of school children,4 as well as increased risk for cardiometabolic disease5 and potentially other health outcomes like diabetes and cancer.6 In 2011, the World Health Organization estimated that 1.6 million healthy life years are lost annually in the western European member states of the European Union alone due to environmental noise.7 Of these, 55.8% of healthy life years lost can be attributed to sleep disturbance, which is the focus of this article.

www.shutterstock.com. Sleep, audiology, noise.

IMPORTANCE OF SLEEP FOR WELL-BEING AND HEALTH

The scientific evidence that sufficient sleep is of paramount importance for well-being and health is overwhelming.8 Sleep is a vital and very active biological process during which, among others, memory is consolidated,9 specific hormones are secreted, and metabolic waste products that accumulate during the wake period are eliminated from the brain.10 In simple terms, sleep is recuperative and prepares us for the next wake period. However, for sleep to be recuperative, it not only needs to be long enough, but it also needs to be of high quality.

NOISE EFFECTS ON SLEEP

Noise is one factor among many that can disturb sleep, reduce sleep quality, impair recuperation, and—if the exposure is chronic—likely contributes in major ways to long-term health consequences like cardiovascular disease. From an evolutionary perspective, sleep is a dangerous state, as we are unconscious and unaware of ourselves and our surroundings. The auditory system plays a critical watchman function, constantly evaluating our environment for threats and arousing us from sleep if necessary. Importantly, our brain is analyzing sounds not only for level, but also for content during sleep.11 For the above mentioned reasons, studies on the effects of noise on sleep that solely rely on self-report are problematic, and there are often major discrepancies between objectively assessed physiologic reactions to noise during sleep and subjective assessments in the morning. Thus, the public health relevance of noise effects on sleep is likely underestimated.

Sleep can be objectively measured with polysomnography, i.e., electrodes attached to the scalp, next to the eyes, and on the chin. These signals are used to differentiate different sleep stages (light, deep and REM) that collectively define the architecture of sleep. Less invasive methods that measure body movements and heart rate can be used to infer sleep architecture. Noise fragments sleep and reduces its continuity by causing intermittent awakenings or shorter arousals. First noise-induced awakenings can typically be observed when the sound level in the bedroom exceeds 30-35 dBA, depending on background noise levels.12 As a consequence, sleep will be shorter and lighter, with less time spent in deep and REM sleep. The latter serve important functions for sleep recuperation. As a consequence, daytime sleepiness increases while performance decreases.13,14 Also, the evidence that sleep is an important mediator of long-term health consequences is increasing.

HABITUATION TO NOISE DURING SLEEP

Subjects exposed to noise usually habituate. For example, the probability that noise causes physiologic reactions is higher during the first nights of a laboratory experiment compared to the last nights,15 and exposure-response relationships derived in the field (where subjects have often been exposed to the noise for many years) are usually much shallower than those derived in laboratory settings, which often include exposure to unfamiliar noise events in an unfamiliar environment.16,17 Habituation is a reasonable mechanism that preserves energy resources. However, habituation is not complete, i.e., subjects continue to react to noise events even after several years of noise exposure. Unfortunately, little is known about individual differences in the ability to habituate to noise and potential predictors.

INTER-INDIVIDUAL DIFFERENCES

Sensitivity to nocturnal noise exposure varies considerably between individuals. Even in a relatively homogeneous and healthy study population, a considerable amount of the variance observed in noise-induced sleep disturbance was explained by inter-individual differences that could not be explained by age, gender, or specific study design aspects.18 There is likely an optimal level of “arousability” that strikes a balance between being aroused from sleep too easily and thus fragmenting sleep and impairing recuperation, or not being aroused from sleep often enough and thus exposing the organism to threats or, e.g., allowing extreme oxygen desaturations in patients with obstructive sleep apnea. Individuals over 65 years old, children, shift workers, and patients with pre-existing (sleep) disorders are considered risk groups for noise-induced sleep disturbance.19 Hospitals are often required to have additional sound insulation to reflect the increased sensitivity of the patient population, but the sound environment in hospitals is far from perfect when it comes to the promotion of undisturbed sleep.20

LONG-TERM HEALTH EFFECTS OF NOCTURNAL NOISE EXPOSURE

Epidemiologic studies have shown an association between long-term exposure to relevant noise levels and cardiovascular disease risk (e.g., high blood pressure, heart attacks, stroke).5,21 A recently published study suggests that the effects of noise on cardiovascular health start with an emotional reaction (indicated by an increased activation of the amygdala, the region of the brain that plays a key role in processing emotions) that then triggers an inflammatory response.22 A recent retrospective case-crossover study at Zurich airport demonstrated that aircraft noise exposure levels in the 2 hours preceding the event were associated with cardiovascular death.23 Thus, nocturnal noise exposure may not only contribute to pathophysiological changes that increase cardiovascular disease risk, it may also evoke physiological arousal that triggers fatal events. The increases in noise-induced cardiovascular disease risk are typically small to moderate (e.g., risk increases by 2%-10% per 10 dB increase in noise exposure). However, this still constitutes an important public health problem, as large parts of the population are exposed to relevant environmental noise levels.

Epidemiologic studies suggest that nocturnal noise exposure is more relevant for the genesis of long-term health outcomes than daytime noise exposure, probably also due to the fact that people more consistently are at home during the night than during the day.24 Two studies found evidence for impaired flow-mediated dilation of the brachial artery after a single night of exposure to rail25 or aircraft noise,26 suggesting that a single night of noise exposure can relevantly affect blood vessels. The latter findings were replicated in a patient population with or at high risk for coronary artery disease.27 Analyses of blood proteins25 demonstrated noise-induced changes indicative of a pro-thrombotic and pro-inflammatory phenotype and provide a molecular basis and biologic plausibility for the increased cardiovascular disease risks observed in epidemiological studies and other disease endpoints like neurodegenerative disease,28 obesity,29 diabetes,29 and breast30 and colon31,32 cancer. However, the evidence on the effects of noise on these non-cardiovascular disease endpoints is only emerging. In one animal study, negative effects on blood vessels and composition were primarily observed if the noise exposure was intermittent and during the sleep phase,33 again highlighting the importance of undisturbed sleep for health

NIGHTTIME NOISE POLICY

Noise policy and regulations are typically set separately for individual noise sources, neglecting that some people are exposed to more than one noise source simultaneously.15 They often differ internationally, and national policies and regulations can be set at the federal, state or local level. They are often informed by guidelines produced by experts in the field and commissioned by entities like the World Health Organization (e.g., Environmental Noise Guidelines).34 One of the main goals of noise effects research is to derive exposure-response functions that can then be used for health impact assessments and ultimately to inform political decision making.2 As the source that produces the noise often also generates either revenue (e.g., air traffic) or pleasure (e.g., outdoor concerts, loud motorcycle exhaust systems), noise policy is a difficult balancing act that needs to weigh both positive and negative aspects of the noise source. The latter include the non-auditory health effects of noise as well as loss of property values.

NIGHTTIME NOISE MITIGATION

Noise mitigation at the source is the best strategy to prevent noise effects on sleep. These measures include traffic curfews that forbid traffic or the generation of noise during the night or parts of the night. However, noise reduction at the source can be technically not feasible or too expensive. Choosing a home in a quiet area should be a priority, but neither this nor moving away from areas that have become more noisy is possible for many, demonstrating a noise equity issue. Noisy areas are typically the cheapest in which to live. Changing the bedroom from a room facing the street to a room that faces the backyard can be a successful strategy for road traffic noise. Passive sound insulation should always be a last resort measure, but it can be effective as we typically spend our time sleeping in the same room.35 Of note, sound insulation windows lose their effectiveness if a window is opened during the night. Thus, in areas with high temperature and humidity during the warm season, it may be necessary to install central air conditioning in addition to sound-insulating windows. However, air conditioning units, especially window units, can be noisy themselves and potentially disrupt sleep.

Ear plugs can be used to lower perceived noise levels. Still, the efficacy of earplugs in preventing sleep disturbance is understudied and poorly understood.36 Also, some people do not tolerate earplugs. Sound and white noise machines for promoting adequate sleep have grown in popularity; yet, a recent systematic review found little evidence for or against the effectiveness of white noise machines and apps.37

CONCLUSIONS

The effects of noise on health go far beyond hearing loss. Undisturbed sleep is a prerequisite for high levels of daytime performance, well-being and health. Environmental noise can disturb sleep and impair sleep recuperation. Nocturnal noise exposure has been associated with multiple negative health outcomes, and basic, translational, laboratory and field research provide biologic plausibility for the causal effects of noise-induced sleep disturbance on health. This demonstrates the growing and often underestimated threat of noise exposure to sleep recuperation and public health.

ACKNOWLEDGEMENT

Parts of this article were published in: Reference Module in Neuroscience and Biobehavioral Psychology, Basner, M.: Effects of noise on sleep. 2021, Copyright Elsevier (with permission).

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