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subjects on the morning after a night’s sleep (Douglas and Murphy, 2016).
Recent studies focusing on the effects of aircraft noise events on sleep structure have shown that they are associated with a decrease in SWS and increased awakening frequency in study subjects (Basner and Samuel, 2005; Basner et al., 2006). The review by Perron et al. (2012) found a clear asso- ciation between aircraft noise events and sleep disturbance. The disturbances varied across studies but generally includ- ed awakenings, decreased SWS time, and the increased use of sleep medication for noise-exposed subjects. This possible relationship was flagged more than four decades ago when it was reported that rapid eye movement (REM) sleep rhyth- micity may also be affected by environmental noise (Naitoh et al., 1975). REM is one of the deep-sleep stages that is im- portant for physical recuperation. Other research by Ohr- strom and Skanberg (2004) has shown that sleep quality at home is reduced after exposure to traffic noise when com- pared with a quiet reference night.
A study using subjects from Gothenburg, Sweden, analyzed the effects of train noise and vibration on human heart rate during sleep (Croy et al., 2013). The results showed a statisti- cally significant change in the subjects’ heart rate within one minute of exposure to train noise and the cardiac responses tended to be higher in the high-vibration than in the low- vibration condition. The results show that the human physi- ology reacts almost instantly to noise exposure during sleep. In this case, the authors concluded that train noise provokes heart rate accelerations during sleep. Similar results have been found in related studies (Griefahn et al., 2008; Tassi et al., 2010).
Noise-induced sleep disturbance can vary for different modes of transport (road, rail, air) or modes in combina- tion. In a laboratory study in Germany, 72 subjects (32 men) were studied for 11 consecutive nights with 0, 40, 80, and 120 noise events employed in a balanced design in terms of the number of noise events, maximum sound pressure level, and equivalent noise load (Basner et al., 2011). The results revealed that road traffic noise was responsible for the most significant changes in sleep structure and continuity despite the fact that the subjects considered air and rail more dis- turbing subjectively; cortical and cardiac responses during sleep were lower for air compared with road and rail traffic. An interesting aspect of the study was that the authors asked subjects to complete morning questionnaires to subjectively assess their previous night’s sleep. They found that despite subjects being in an unconscious state for most of the night,
they were capable of distinguishing not only between nights with and without noise but also between nights with low and high degrees of traffic noise exposure. This and related work (see Douglas and Murphy, 2016) imply that morning ques- tionnaires might be a more robust method of assessing traf- fic noise effects on sleep than previously thought.
One of the major issues related to environmental noise and sleep disturbance concerns how the noise might be charac- terized, such as whether the noise is continuous or intermit- tent. Laboratory studies using recorded intermittent and continuous traffic noise have demonstrated beyond any rea- sonable doubt that human subjects are more disturbed by intermittent noise than by continuous noise (see Ohrstrom and Rylander, 1982; Murphy and King, 2014). In Ohrstrom and Rylander’s study, subjective sleep quality, mood, and performance on reaction time tasks were all impaired by ex- posure to intermittent environmental noise at night, where- as continuous noise had considerably less impact on sleep quality and no impact at all on mood or task performance. A different study found that intermittent noise with peak levels above 45 dB(A) can increase the time taken to fall asleep by up to 20 minutes (Ohrstrom, 1993). And yet, for public health purposes, noise continues to be evaluated during the nighttime with continuous equivalent noise level indicators such as the sound pressure level in decibels. For example, Leq is equivalent to the total sound energy over a given period of time. In Europe, Lden is the A-weighted day-evening-night Leq. It is the noise level measured over a 24-hour period, with a 10-dB penalty added to the level between 2300 and 0700 hours and a 5-dB penalty added to the level between 1900 and 2300 hours. Lnight is the A-weighted nighttime (2300 to 0700 hours) Leq. Lden and Lnight smooth out intermittent noise events but, more importantly, underestimate the magnitude of the disturbance in favor of the polluter.
In the animal kingdom, there are even more worrying paral- lels. An important 2005 study tested rats to determine the ef- fect of chronic exposure to environmental noise on sleep and to evaluate the interindividual vulnerability of sleep to envi- ronmental noise (Rabat et al., 2005). The study monitored the sleep states of the rats by EEG recording and chronically implanted cortical electrodes. The results of the study dem- onstrated that after nine days of environmental noise expo- sure, there was an increase in wakefulness, amounting to 16 hours when compared with a controlled environment of 40 dB(A). In addition, the results showed that exposure dis- turbed both SWS and paradoxical sleep (PS); after 9 days of exposure, rats lost about 1.1 and 0.75 h/day of SWS and PS,
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