Assessment Of Noise-induced Sleep Fragility In Two Age Ranges By Means Of Polysomnographic Microstructure

Abstract

The microstructure of sleep, which translates the short-lived fluctuations of the arousal level, is a commonly neglected feature in polysomnographic studies. Specifically arranged microstructural EEG events may provide important information on the dynamic characteristics of the sleep process. CAP (cyclic alternating pattern) and non-CAP are complementary modalities in which arousal-related "phasic" EEG phenomena are organized in non-REM sleep, and they correspond to opposite conditions of unstable and stable sleep depth, respectively. Thus, arousal instability can be measured by the CAP rate, the percentage ratio of total CAP time to total non-REM sleep time. The CAP rate, an age-related physiological variable that increases in several pathological conditions, is highly sensitive to acoustic perturbation. In the present study, two groups of healthy subjects without complaints about sleep, belonging to different age ranges (six young adults, three males and three females, between 20 and 30 years, and six middle-aged individuals, three males and three females, between 40 and 55 years) slept, after adaptation to the sleep laboratory, in a random sequence for two non-consecutive nights either under silent baseline (27·3 dB(A) Lcq) or noise-disturbed (continuous 55 dB(A) white noise) conditions. Age-related and noise-related effects on traditional sleep parameters and on the CAP rate were statistically evaluated by a split-plot test. Compared to young adults, the middle-aged individuals showed a significant reduction of total sleep time, stage 2 and REM sleep and significantly higher values of nocturnal awakenings and the CAP rate. The noisy nights were characterized by similar alterations. The disruptive effects of acoustic perturbation were greater on the more fragile sleep architecture of the older group. The increased fragility of sleep associated with aging probably reflects the decreased capacity of the sleeping brain to maintain steady states of vigilance. Total non-REM sleep described by traditional parameters was statistically unaffected during the disturbed nights, but the perturbing effects of noise on non-REM sleep stability and continuity were revealed by a significant increase in the CAP rate. The perspectives for a wide-ranging exploitation of this sleep parameter are discussed.