- Understanding aviation noise
- Health impacts of noise
- Sleep disturbances
Undisturbed sleep of sufficient length is a vital process for human beings, which is providing the necessary daytime alertness, performance ability and health. A good night's sleep is essential for a wide array of vital functions, such as:
- cognitive and neurobehavioral functioning;
- consolidation of memory contents (storage of experiences in long-term memory, solidification and separation of nerve connections);
- the immune system;
- mood regulation; and
- metabolic regulations, such as sugar metabolism (Basner & McGuire, 2018).
During sleep time, human beings are usually unconscious. However, the ear has an alerting function to prevent harm from possible ambient threats and, therefore, continually inspect the environment acoustically whilst sleeping.
Within the last decades, the growing need for mobility and transport of goods has led to increasing night noise, potentially identified as such a threat by some individuals and can lead to disturbed sleep amongst other outcomes. The consequences of such interruptions of sleep from transport noise can be classified as follows:
- immediate reactions can cause additional awakenings, palpitations and reduced deep and Rapid Eye Movement (REM or 'dream') sleep during the night;
- short-term responses during the following day can be fatigue, lack of concentration and, therefore, a higher risk of accidents and reduced quality of life perception as well as the feeling of annoyance;
- long-term consequences after years of permanent night traffic noise can increase the risks for high blood pressure, ischaemic heart diseases and blood vessels dysfunction, as well as metabolic and mental disorders such as depression.
The connection between immediate and long-term effects is still unclear and under research. However, experimental studies investigating the impact of noise at night are usually very accurate in measuring the perceived noise and the physiological variables to be examined. Moreover, they can avoid or at least control the influence of potential non-acoustic factors that may bias the association between noise exposure and its physiological effects. Results of these studies suggest that a concept for protecting the population against noise at night must be designed to avoid noise's immediate effects. In this sense, according to current knowledge, potential health risks will be minimised after many years of exposure.
How can sleep disturbances be measured?
A vast number of studies examined the effects of environmental noise on sleep, and there are different methods to measure sleep.
- The most comprehensive methodology to physiologically assess sleep is so-called polysomnography (PSG), which encompasses various measurements, including electrophysiological assessment of brain activity (electroencephalogram, EEG), eye movement (electrooculogram, EOG), heart activity (electrocardiogram, ECG), as well as muscle activity (electromyogram, EMG).
Figure 1: Schematic representation of the electrode positioning for the measurement of polysomnography (source: DLR)
Figure 2: Preparing subjects for polysomnographic recordings (source: DLR)
- A less invasive method for participants is actigraphy, which measures a person's body movements via a single device worn on the wrist. In recent studies, authors attempted to use the combination of actigraphy and ECG measurements in order to detect awakenings in a less costly way than employing PSG.
- Next to physiological measurements, there is also the option to ask participants about their experience of sleep disturbances or sleep quality, i.e. a psychological assessment of sleep. The items used for this type of assessment can refer to the last night's sleep or a participant's sleep experience over a more extended period. Estimates using questionnaires are relatively cheap and easy to implement. Numerous published examples have been used for calculating 'mean' exposure-response curves of 'at least a little sleep disturbed', 'sleep disturbed' and 'highly sleep disturbed' people over an energy equivalent noise level LNight. In March 2020, the new Directive EU 2020/367 was published to have a rough idea of this disorder across the EU countries. However, research has shown that psychological assessments do not necessarily reflect physiologically measured sleep disturbances and interruptions.
The central gap of these curves, based on equivalent levels, is that those 'mean' levels, if not unrealistically low, are not suitable for communication to communities or protection purposes for noise effects on sleep as many different noise situations can lead to the same Leq but have other effects on sleep. The World Health Organisation recommends in its "Environmental Noise Guidelines for the European Region" (2018) an LNight threshold of 40 dB(A) for aircraft noise due to the number of 'Highly Sleep Disturbed people' (HSD), assessed by questionnaires (WHO, 2018). In reality, a Boeing 747 on take-off with a distance of 10 km from the runway, 600 m height, one movement per 8 hours already would lead to an LNight of 51 dB(A), or for an Airbus 320, 10 km distance from the runway on take-off, 770 m height, eight movements per 8 hours would lead to an LNight of 50 dB(A).
With this information in mind, to develop a night noise protection concept, it seems advisable to revert to (1) physiologically measures sleep assessment techniques and (2) to acoustical parameters of single aircraft noise events, such as maximum levels, event duration, and level rise speed, instead of referring to equivalent levels for the whole night. This technique focuses on the relation between the acoustic parameters of single noise events during sleep time and reactions in sleep, particularly awakening reactions due to the noise event. This so-called event-related analysis also allows the development of exposure-response models and, thus, estimates the number of additional awakening reactions because of aircraft noise that serves as the basis for night noise protection concepts. Therefore, despite their complex and laborious assessment, the advantages of physiological measurements of sleep disturbances and interruptions against self-assessments become apparent.