Sleep spindle characteristics and arousability from nighttime transportation noise exposure in healthy young and older individuals

Study Objectives: Nighttime transportation noise elicits awakenings, sleep-stage changes, and electroencephalographic (EEG) arousals. Here, we investigated the potential sleep-protective role of sleep spindles on noise-induced sleep alterations. Methods: Twenty-six young (19-33 years, 12 women) and 18 older (52-70 years, 9 women) healthy volunteers underwent a repeated measures polysomnographic 6-day laboratory study. Participants spent one noise-free baseline night, followed by four transportation noise-exposure nights (road traffic or railway noise; continuous or intermittent: average sound levels of 45 dB, maximum sound levels of 50-62 dB), and one noise-free recovery night. Sleep stages were scored manually and fast sleep spindle characteristics were quantified automatically using an individual band-pass filtering approach. Results: Nighttime exposure to transportation noise significantly increased sleep EEG arousal indices. Sleep structure and continuity were not differentially affected by noise exposure in individuals with a low versus a high spindle rate. Spindle rates showed an age-related decline along with more noise-induced sleep alterations. All-night spindle rates did not predict EEG arousal or awakening probability from single railway noise events. Spindle characteristics were affected in noise-exposure nights compared to noise-free nights: we observed a reduction of the spindle amplitude in both age groups and of the spindle rate in the older group. Conclusions: We have evidence that spindle rate is more likely to represent a trait phenomenon, which does not seem to play a sleepprotective role in nighttime transportation noise-induced sleep disruptions. However, the marked reduction in spindle amplitude is most likely a sensitive index for noise-induced sleep alterations. Statement of Significance Growing epidemiological evidence suggests various adverse health effects of long-term nighttime transportation noise exposure. In the EU, 7.9 million adults are estimated to experience sleep disturbances due to nighttime transportation noise exposure with additional community noise sources such as neighbor noise not even considered. Individuals differ greatly in sleep-related noise sensitivity. A better understanding of the mechanisms that underlie these differences could help to identify targets for intervention. This study in young and older healthy individuals indicated that sleep spindles, discussed in the literature for their potential sleep-protective function, do not significantly modify noiseinduced sleep alterations after controlling for age and may therefore not serve as a physiological marker of sleep-related noise sensitivity.