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|Year : 2010 | Volume
| Issue : 47 | Page : 70--76
Sleep disturbance due to noise: Current issues and future research
Division of Health Science and Centre for Air Transport and the Environment, Manchester Metropolitan University, United Kingdom
Division of Health Science & CATE, John Dalton Bldg., MMU, Chester St., Manchester M15GD
There is growing interest in carrying out further research to understand and reduce the impact of aircraft noise on airport neighborhood in anticipation of the projected substantial increase in global aviation. Soundscapes provide new analytical methods and a broader, more comprehensive appreciation of the aural environment, which may have a useful role in understanding noise-induced sleep disturbance and annoyance. Current noise metrics like Leq do not provide a common language to report noise environment to residents, which is a key obstacle to effective noise management and acceptance. Non-auditory effects complicate the production of consistent dose-response functions for aircraft noise affecting sleep and annoyance. There are various end-points that can be chosen to assess the degree of sleep disturbance, which has detracted from the clarity of results that has been communicated to wider audiences. The World Health Organization (WHO-Europe) has produced Night Noise Guidelines for Europe, which act as a clear guide for airports and planners to work towards. Methodological inadequacies and the need for simpler techniques to record sleep will be considered with the exciting potential to greatly increase cost-effective field data acquisition, which is needed for large scale epidemiological studies
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Hume K. Sleep disturbance due to noise: Current issues and future research.Noise Health 2010;12:70-76
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Hume K. Sleep disturbance due to noise: Current issues and future research. Noise Health [serial online] 2010 [cited 2020 Sep 27 ];12:70-76
Available from: http://www.noiseandhealth.org/text.asp?2010/12/47/70/63206
Sleep is a fundamental human behavior which is essential for development, health and well-being. Sleep displays both homeostatic and rhythmical features and its alternating pattern with wake presents one of the most obvious and essential human circadian rhythms. Sleep is a reversible process which can be readily disturbed by noise to cause a full range of disturbance from full awakening to minor unconscious autonomic perturbations. This reversible nature of sleep due to noise has provided an essential evolutionary advantage where the auditory system, unlike the visual system, can still monitor the environment and arouse the vulnerable sleeper in response to potential danger. In present times, noise-disturbed sleep is a cause of considerable annoyance with potential health and well being affects.
Noise pollution has been described as the 'modern unseen plague.' It is generally accepted that the developed and developing countries are becoming noisier places. For example, mobile phone ring tones have the ability to disturb the most tranquil place at any time of day or night. Humans are responsible for most of the noise that disturbs sleep in the home, whether it is generated outside due to transportation noise or inside because of a loud snoring partner; both are unwelcome.
Noise can have a number of unwanted effects. It reduces the fidelity of communication, interferes with cognitive processes and disturbs sleep, all of which can contribute to cause annoyance and complaint. There are various metrics and standards that have been modeled and applied to environmental noise in an attempt to limit the problem to an acceptable degree. Many research workers in the field consider sleep disturbance due to environmental noise to be the most detrimental to health. Most people would consider the availability of an undisturbed night's sleep as a fundamental right and a prerequisite for their continued health and well-being.
The perceived problem of noise disturbance depends, to a large part, on non acoustic factors. For example, the extent of nuisance perceived by an individual depends on their relative position along a utility perspective with regards to the noise producer. An airport worker who lives near an airport may well be affected by aircraft noise but s/he is far more likely to accept this nuisance and cope with the stressor than a neighbor who does not rely on the airport for their livelihood. This is an example of the many non-acoustical factors that make the production of consistent 'dose-response' curves elusive in this general area. There are varying reports from different countries on the most disturbing form of noise pollution at night, for example, mopeds in The Netherlands, but transportation noise at night is a major factor in most reports. A reviewer  has echoed a frequent observation in 'annoyance and complaint analysis' that the "psychological dimension of the expressed annoyance is highly related to the specific relationship that exists between the noise producer and receiver," in essence, your neighbor's dog barks louder than your dog! It is evident that non-auditory factors (e.g. relationship with the noise producer) play a significant part in the subjective response to sound and noise and the degree of disturbance or annoyance reported.
Noise is generally considered 'bad' but is frequently a by-product of well-accepted operations which are valued by the community at large. For example, the traveler considers rapid transportation as 'good' while the resident near the highway, train track or airport considers the noise a nuisance i.e. 'bad'. This is similar to many controversial issues in the world today where there is a need for balance between the potential damage that noise can cause in a neighborhood and the benefits accrued by many people from the modern transport systems. In such situations, the scientists need to assess the point at which unacceptable harm is likely to occur and provide suitable metrics which allow appropriate regulation and management.
There is considerable current interest in carrying out further research to understand and reduce the impact of aircraft noise on airport neighborhood in anticipation of the projected substantial increases in global aviation. There is a realization that the standards and criteria applied to determine environmental impact need updating because of the changes that have occurred in aviation over the last few decades, such as quieter aircraft but a greatly increased number of flights. Also, awareness of environmental issues and the expectations of a better quality of life seem to have risen in the general public. The US Department of Transportation Federal Aviation Administration (FAA) apply a level of 65 DNL (LaMax.A) which is the point at which 25% of residents on a composite curve were predicted to become extremely annoyed by aircraft noise.  The cut-off point is arbitrary but clear-cut and provides a level which is embedded in current US legislation. However, the FAA is currently in the process of developing a research program to review this standard and its applicability for the future which scientifically informs future policy to reduce the impact of aircraft noise.  The European Union Directive 2002/49/EC (END) requires member states to generate environmental noise maps which are decibel/Leq-based, irrespective of the sound components of this noise and whether they are considered annoying or not. However, besides a requirement to generate 'action plans' from these noise maps, there is no practical guidance provided for assessing and mitigating the potential effects of traffic noise on sleep.
Part of the increase in global noise is due to continued growth of a 24 hour culture in most developed countries which results from both more activity and noise intrusion into the night-time sleep period and more people trying to sleep during the daytime (e.g. shift workers) when ambient noise levels are higher. Not only is the environment becoming noisier but it also seems, from some annoyance data, that our 'ears are becoming bigger' as we are becoming more sensitive and annoyed at lower noise levels, particularly by aircraft noise.  This effect has been attributed to increased affluence and expectation of higher qualities of life and is more noticeable where step changes in the noise source have occurred, e.g. initial operation of a new runway. 
This brief review will consider some fundamental issues concerning noise and sleep disturbance, consider the continuing and emerging issues with some suggestions how research may develop in the future, with an emphasis on transportation noise and particularly aviation.
Sound Level and Soundscapes
There have been recent attempts to understand the cause of noise disturbance better, the realization that it is not just the noise level, as usually measured in decibels, but a more complete analysis and description of the noise, in the way that ones favorite music can be played at much higher sound levels without apparent distress is required. This could involve new analytical methodologies associated with soundscapes, which provide a broader and more comprehensive appreciation of the aural environment. 
It has been demonstrated that more rapid rise times in the sound envelope for an aircraft noise event increases the ability to awaken a sleeping subject  and /or induce a movement arousal.  Sounds in which various fundamental characteristics were altered (e.g. square waves, mixtures of pure tones, white noise) have recently been shown  to have a 20 dB difference in their ability to awaken sleeping subjects.
The soundscape approach tries, in part, to provide a wider, more comprehensive description of noise and sound. For example, it can be seen how extremely limited it would be to describe a piece of music just by sound level indicators. An example of this shift to a more comprehensive description of noise is seen in the recent European involvement in an International COST project (2008) entitled 'Soundscape of European Cities and Landscapes' in which its main objective is stated as "to provide the underpinning science for soundscape research and make the field go significantly beyond the current state-of-the-art, through coordinated international and interdisciplinary efforts." This well funded initiative accepts that "reducing sound level ... does not necessarily lead to improved quality of life in urban/rural areas, and a new multidisciplinary approach are essential." It indicates that the cooperation of a broader mix of specialism including human and social sciences is needed apart from physical measurement to fully understand the effects of sounds.
There have been European initiatives to understand the underlying community impact which involved investigating how to make the sound of aircraft more pleasant, for example, the recent EU-funded study SEFA (Sound Engineering For Aircraft, 2003-07) in which aircraft noise producing components were investigated for potential modification to produce an overall aircraft sound which was not necessarily quieter but produced a more acceptable sound signature to the human ear. These techniques have been applied successfully in the car industry, but this was primarily for the benefit of the driver and passengers and not the community and roadside pedestrians hearing the car pass-by. SEFA has been recently followed by a closely related European project COSMA (Community oriented solutions to minimize aircraft noise annoyance, 2009-2012) which has as its ultimate goal - the design of aircraft and their operations for a minimum of annoyance in the surrounding airport community.
A major cause of annoyance and complaint around large noisy operators like airports seems to be due to the lack of transparency and understanding between the way airports and governmental organizations provide information on the noisiness of an airport. Noise information is usually provided in time period energy equivalent noise contours; for example, Ln or DNL, which is a number of transpositions away from what the residents actually hear and is not some averaged cumulative noise metric but individual aircraft noise events. Supplementary metrics in Australia  have made a positive contribution to consultation exercises. There have been recent studies in the UK  that highlighted these problems and the benefits of clearer metrics which better reflect the resident's experience. For example, the number of minutes per day that aircraft noise exceeds a noticeable level, like, N70.
In the UK study,  focus groups revealed that residents were confused and distrusted energy equivalent metrics such as Leq which are aggregations of individual aircraft noise events (ANE). They favored disaggregation of key elements i.e. time, frequency of flights and noise level; even preferring bar charts for specific location sites and specific information such as flight path densities overlaid onto noise foot prints e.g. N70 and Leq. There was some appreciation that the conventional Leq metric allows a legally defendable basis for land planning and developmental decisions but overall inhibits public scrutiny and understanding and does not provide a common language for reporting the noise environment, which is a key obstacle to effective noise management and residents acceptance.
Noise and Sleep Disturbance
Sleep is the paramount restorative process necessary to maintain normal levels of brain and behavioral functioning, mood and well-being while awake. Sleep disturbance is important because if it is sufficiently severe it reduces our nightly recuperation, which then affects our waking performance as well as our health and mood. There are limited techniques available to measure the effect of sleep loss or disruption on waking activities and they are cumbersome and time consuming; for example, multiple sleep latency test (MSLT) and vigilance tasks. More recently, there has been interest in daytime studies that are more relevant and applied which highlight previous night's sleep problems, like driving error in simulators.
There is further complexity in sleep where noise can affect the cardiovascular system without producing full arousal i.e. awakening. Therefore, sleep disturbance is not a unitary concept; there is a full range of effect from full blown behavioral awakening signaled by the subject via a push-button, to subtle changes in autonomic physiology; these changes are not necessarily consistent within an individual for a given level of noise stimulus as there are complex patterns of neurophysiology associated with the different EEG-defined sleep stages and the time of night. In addition, actigraphy, which measures limb movement has been used as it is frequently associated with relatively major arousals in the sleep EEG. Also, autonomic nervous system arousals like heart rate accelerations, which can occur in response to noise without major EEG arousals, have been used to gauge sleep disturbance. Given this mix of techniques it is easy to see that there are various end-points that can be chosen to assess the degree of sleep disturbance. However, measures are extracted from EEG based polysomnography, which is considered the 'gold-standard' of sleep recording and provides a direct measure of cerebral activity from which a number of macro and micro-structural features can be extracted. 
A number of reviewers have pointed out that this diversity of end-points has detracted from the clarity of results that can be communicated to wider audiences. In this short paper it is impossible to fully review all the relevant work on sleep disturbance due to noise and not necessary when adequate longer reviews exist. However, it is possible to consider the main important consensus findings and understand the pressures operating, like political, social and economic; based on current evidence, useful suggestion and pointers can be made for the future.
A major development over the last five years in this field has been the full publication of the largest and most comprehensive study carried out by Alex Samel and Mathias Basner and co-workers at the DLR-Institute of Aerospace Medicine in Germany, which has provided a wealth of information on the effect of aircraft noise on sleep. They recorded a total of 2,240 subject nights in both the laboratory and field. This database provided clear results in terms of changes in the macrostructure of sleep stages, immediate event related analysis, dose - response relationships between aircraft maximum sound pressure levels and the probability of awakening.  The DLR group also applied their findings directly to the difficult practical problems of noise disturbance around busy airports and developed the concept of 'noise protection zones' on the basis of sleep disturbance rather than the traditional noise contours which are based solely on acoustic criteria. 
The World Health Organization (WHO - European Office) has been instrumental in bringing experts together in recent years and preparing documents with focus on establishing Night Noise Guidelines for Europe,  Aircraft Noise and Health and Practical Guidance for Health Risk Assessment of Environmental Noise in Europe, which contain up-to-date reviews of noise and sleep disturbance and the potential risk to health. The WHO, NNG summarize the relationship between night noise and health effects in the population into four ranges of continuous outside sound level at night Ln:
55dB - adverse health effects occur frequently with high percentage of the population highly annoyed.
These guidelines are a bold first step to lay down some easily understood guidelines based on research carried out mainly in Europe and provide a clear and simple guide for airports and planners. The fuller understanding of the effect of noise on sleep depends to a large extent on more fully understanding the fundamental questions of the nature and function of sleep. For example, it is now emerging that besides its major restorative function it is suggested that sleep plays an important role in brain plasticity mechanisms and work on memory consolidation during sleep indicates the roles of SWS for explicit contents and REM for implicit contents has shown considerable advance in recent years. Therefore, the more we understand what undisturbed sleep does, the more we will understand what deficits will be incurred when sleep is disturbed.
One direct advance in sleep research which benefits our fuller understanding of the effects of noise on sleep is work into establishing the level and nature of normal values of spontaneous arousals and perturbations that occur in sleep,  which allows clearer assessment and identification of what may be a significant increase due to some potential sleep disturbing factor such as noise. Also, studies directed at understanding the essential link between EEG arousals, sleep fragmentation and reduced daytime functioning. 
The most generalized disturbing noise in the urban and suburban environment, where most of the populations in the developed world live, is transportation noise, particularly due to road, rail and specifically air traffic. This was again reflected at ICBEN 2008, where as in previous years, transportation noise and particularly aircraft noise dominated concern and research affecting sleep.
Besides the acute waking and arousal affects on sleep due to noise that can be monitored via polysomnography, actimetry and behavioral response, there are potential chronic additive effects of nightly noise which have the potential to contribute to cardiovascular disease (CVD). The HYENA (Hypertension and exposure to noise near airports) study  conducted around six major European airports found a significant odds ratio for the risk of hypertension being associated with the night noise level (Ln). This and other studies provide sufficient evidence for a positive association between aircraft noise and cardiovascular disease but because of a lack of consistent end-points and uniform methodologies it is difficult to arrive at a threshold level for pathology onset and a simple dose-response curve. It has to be remembered that there are well over a 100 putative risk factors identified for cardiovascular disease which make any rigorous investigation and analysis of the etiological pathway a difficult task. There have been suggestions in the literature that annoyance is the mediating factor between noise exposure and CVD with annoyance has associations with a number of cofactors like noise sensitivity, negative affectivity and mental health, which fit stress models in that the inability to cope with the environmental noise stressor is the crucial etiological link.  It should not be forgotten that there are internal as well as external causes of sleep disturbance. Internally, there are various sleep pathologies like sleep apnea, somatic illnesses like. Infections and factors of a more psychological origin like anxiety and stress which are frequently work related. So there is a need to appreciate the additive disruptive aspect of environmental noise and incipient sleep pathology as sleep disorders are common >10% in the general population and increase with age, with many going undetected because of lack of awareness by community doctors and the general public. This presents an additional difficulty in ascribing sleep disturbance to a specific external cause and emphasizes the case for adequate screening and internal control measures for subjects.
Continuing and Emerging Issues
There are many indications that the number of people exposed to transportation noise and particularly aircraft noise disturbance will increase over the next 20-30 years. There have been substantial increases in aircraft noise over the last five years, however, the recent global economic slow-down may reduce the rate of growth for a year or two. Concern and action to limit aircraft carbon emissions in the future may further complicate future predictions as noise reduction usually has an energy cost.
Europe and other regions have witnessed a marked increase in low cost 'budget' air operations over recent years, which has had the effect of increasing air travel and, due to limited capacity at the major airports, this market has tended to move to smaller regional airports. This has increased the number of flights at these traditionally 'quiet' airports and, importantly, they tend to have a much lower ambient noise level compared to the major airports which are usually found in larger conglomerations. Therefore the potential for aircraft noise disturbance has increased and spread.
Despite the majority of larger European airports being subjected to noise related capacity constraints, many airports are developing, wherever possible, to maximize their customer throughput and profit. Additionally, with the 'awakening of sleeping giants' in Asia (China and India), global travel is set to rise at a considerable rate as economic prosperity and affluence is linked with industry, trade and travel. In parallel to the recent and projected increase in aircraft noise disturbance there are increases in the expectations of the quality-of-life which is seen in the heightened sensitivity and reduced tolerance in noise-affected communities.
As a consequence of the complexity that contributes to people's perceptions and response to disturbance, acoustic variables (e.g. Leq noise contours), on their own, have not been able to satisfactorily predict annoyance and complaints regarding environmental noise. The problem of aircraft noise involves a complex interaction of a number of non-acoustic factors including psychological and sociological issues. As a result there are a number of research groups investigating the non-auditory affects of noise like annoyance.
In the past there has been considerable technical improvement like changes in engine designs that have led to reduced noise; however, the potential for substantial, further reduction in engine noise seems limited. On the contrary, new technologies aimed at counteracting carbon emission and global warming lead to increase in noise. For example, the "open rotor engine," may achieve development in 10-20 years and is projected to give significantly more fuel efficient and less carbon dioxide per air mile, but more noise. So, the noise for the relatively few people who live near airports may have to increase for the good of the world's population as a whole.
In terms of methodological issues concerning noise and sleep disturbance: EEG-based studies have remained the gold-standard as sleep is a phenomenon uniquely associated with the brain whose gross activity is directly measured by EEG. REM sleep needs EOG and EMG additionally for correct classification, but such polysomnography is expensive; ECG and other autonomic measures are useful in determining cardiovascular responses to noise stimuli and can provide insights into disease etiology; actimetry is a convenient adjunct, cost-effective, easy to use and analyze, but its interpretation is not precisely clear; signaled awakening again provides a simple and convenient technique but can be prone to problems of compliance and lacks sensitivity; sleep logs and questionnaires provide useful subjective data and complaints have motivation issues but provide constant feed-back and are key drivers in the political arena.
Field research into the effects of noise on sleep is essential for realism while laboratory studies can deliver appropriate high levels of control of confounding variables that are usually present in the field. Therefore both these types of study are important. Early field studies showed much less sleep disturbance than predicted by laboratory results. This has been explained by a lack of habituation in the laboratory setting. 
Despite all the work in this area there is still uncertainty as to the long term health consequences of night-time noise disturbance on exposed populations. Some authors have commented on the lack of an epidemiological study that shows a causal link between (aircraft) noise, sleep disturbance and long-term illness. So, there is still a need for large scale field studies with representative samples of the population to investigate the association between night-time aircraft noise exposure and cardiovascular disease. The cost of large scale EEG-based studies is considerable and a more cost-effective technique is needed.
A new technique being developed as an asset in the field of noise and sleep disturbance is an ECG-based algorithm for the automatic identification of autonomic activations associated with cortical arousals, which saves considerable human analysis time and aids consistency and objectivity.  A new methodology which utilizes and combines relatively simple yet robust signals such as ECG and actimetry may provide cost effective tools.
Looking to the future, there is potential for exciting development to greatly increase our ability to access considerable field data due to the greatly increasing number of households linked by broadband internet services and WiFi in developed countries; for example, US and Western Europe. This could allow residents from wide areas like around airports, to pass digital information when they would normally be off-line as in bed at night, to a central receiver and analysis point which could also collect co-terminus noise and flight data. There would have to be some development and suitable interfacing to transmit, like electrophysiological data, actimetry and simple subject signaling like time of attempting to sleep. Therefore, ECG and actimetry combine to present themselves as robust and easily recorded signals that would be ideal for this system and, in combination with new analysis algorithms, utilizing these simple and relatively inexpensive techniques, combine with subject signaling. All these need to be developed to provide a new cost effective sleep recording technique suitable for epidemiological studies.
Delayed sleep onset and premature awakening can be extremely annoying experiences and are frequently complained about. Community complaints and annoyance have been key drivers in the political noise arena. There has been some work to systematically analyze complaints and annoyance in high noise areas to better understand these issues. The basic rationale is that the data is provided free, provides a rapid feedback and reflects specific area annoyance. This provides a better understanding of how noise affects individuals and provides evidence for pilots, airlines and airports to modify operating systems to reduce noise exposure in affected areas. More use could be made of complaints if airports ensured that there was (1) adequate awareness of the complaint system (2) ease of inputting a complaint with standardized formats and procedures (3) logging, response and reporting procedures (4) application of GPS (5) use of simple metrics to allow comparison within and between airports.
The problems of road, rail and air traffic have become major issues at the local level with noise and air quality and at the global level with carbon emission and global warming.  There has been significant work completed on combined transportation modes and how additive the disturbance was addressed in papers at ICBEN 2008. In the future the combined effects of noise exposure with other agents like poor local air quality on health needs to be further addressed.
Wind turbines as alternative power sources to carbon fuels have developed at a much more rapid pace than was foreseen five years ago. They have considerable potential to add to the noise burden and disturb sleep. Fortunately, until now, these wind-farms have generally been sited in very exposed locations on the tops of mountains and at sea i.e. well away from the main areas of human habitation. However, they pose a potential threat to sleep as they are relatively permanent structures and do not fly away.
The study of the increased risk in vulnerable groups including young, old, noise sensitive, sleep disorder patients, shift workers is frequently mentioned but rarely studied. Major airports operate 24 hours for seven days a week and employ considerable numbers of local staff who work night shifts; hence it is surprising that this clearly highly vulnerable group, who one would surmise have disturbed sleep, have not been studied in depth to the authors knowledge.
Looking to the future and the projected increased longevity of westernized culture due to improved life-style and healthcare, there are predictions that the proportion of the elderly in society will grow significantly over the next 30 years. This suggests that the elderly, with their more fragile sleep, will become a major vulnerable group as hospital patients, nursing and care home residents. There is a literature based around noise and sleep problems associated with hospitalization, acute/intensive care units and institutionalization particularly in homes for the elderly. Koch et al.  report that adopting a multidisciplinary approach combining noise reduction, promotion of daytime activity and reduction of night time nursing care were the most effective means of promoting sleep; the long term use of sedatives is questionable practice and overuse reduces the quality of life of older people. As we all get older it is in our interest to help provide the knowledge to aid sleep and improve the quality of wakefulness in the elderly.
Another vulnerable group, affected by sleeping in noisy periods near airports, is children who are in bed during the 'shoulder hours' of airport operations, which is the hour or so before and after the night curfew restrictions are in force. This is a time of increased aircraft movement and it is typically a time when children are in bed in the evening and morning. These and many other issues would benefit from properly planned, funded and executed research studies in the future.
The essential part of this work was initially presented at ICBEN 2008, and updated for presentation at Internoise and Euronoise 2009.
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