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Sleep patterns are changing, and daily variations in sleep/wake cycles are common. When researchers assess the sleep pattern of subjects over several days or weeks, their sleep diary, actigraphy, or recently developed wearable device is helpful to measure daily variation in sleep/wake cycle for each individual.1 Collected sleep data of several days or weeks is usually averaged out to know the overall sleep pattern of each individual. Therefore, in numerous researches about the association between sleep and physical or mental health, mean values of sleep indices, such as mean sleep latency, mean sleep efficiency, or mean total sleep time of each individual are usually calculated and used. However, we should consider that mean is just one of representative values and not the exact value itself. Intra-individual night-to-night variability should be considered when we assess sleep pattern with regard to certain phenomena which also fluctuate daily.2 Extensively studied, it is well known that noise influences the physical and mental health. Environmental noise affects a person’s mental health,3 and night time road traffic noise increases the risk of cardiovascular complications.4,5 It is also reported that noises related to road traffic, railways, and airplanes can disturb ones sleep.6,7 Various noises especially disturb sleep of inpatients at hospitals.8,9 In previous literatures, values of noise index were usually averaged out to present the status of environmental noise. However, since the daily variation in environmental noise is common, the mean values of noise indices are not the exact noise level at a certain time. The aim of this study was to explore whether mean sleep indices can reflect the intraindividual night-to-night variability with regard to environmental noise. Received: November 21, 2016 Revised: December 10, 2016 Accepted: December 12, 2016

Environmental Noise and Sleep Disturbance: Night-to-Night Variability of Sleep/Wake Pattern