In each non-REM (NREM) sleep stage, the aggregation of the arousal-related phasic events permits identification of periods of arousal fluctuation (cyclic alternating pattern or CAP) and periods of long-lasting arousal stability (non-CAP or NCAP). As the ratio CAP time to NREM sleep time (CAP/NREM) measures the instability of arousal during sleep, any perturbing event determines an increase of CAP/NREM. On the basis of these premises, 6 healthy volunteers underwent 5 sleep recordings at increasing intensities of sound pressure level (basal condition followed by continuous white noise at 45 dBA, 55 dBA, 65 dBA and 75 dBA, respectively). Besides a remarkable enhancement of CAP/NREM (P less than 0.00001), acoustic perturbation induced a significant linear increase of waking time after sleep onset, stage 2, NREM sleep, stage shifts and a significant linear decrease of stage 4, deep sleep, REM sleep and total sleep time. At each step of environmental disturbance, the values of the CAP ratio were consistent with the gradual changes of sleep organization. Although the Multiple Sleep Latency Test was unremarkable during the day following the sleep recording, CAP/NREM was significantly correlated with the personal evaluation of sleep quality (P less than 0.01). Through this model of transient situational insomnia it was possible to outline different degrees of subjective complaint depending on 3 ranges of CAP/NREM. A crucial role of CAP in the pathophysiological mechanisms of clinical insomnia is hypothesized.
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