Sympathetic arousal and arousability predicts subsequent sleep quality

The aim of this study was to evaluate whether individuals with primary insomnia, in comparison to normal sleepers, show consistently higher autonomic arousal levels or changes in autonomic arousal which is systematically associated to differences in the quality of their night-time-sleeping. To address this aim, skin conductance levels (SCL) and inter beat intervals (IBI) were monitored before and after five consecutive nights in two different experimental conditions: 3 min with the instruction to rest and 3 min while providing acoustic stimuli inducing an orienting reflex. Twenty-three participants were recruited for the study, twelve people with insomnia and eleven good sleepers. The selection of the participants consisted in a first screening phase followed by a clinical interview and two weeks of assessment through the use of sleep diaries. Groups were matched for gender and age. Physiological indices were recorded in the participants' own homes through portable devices for one week (weekends excluded), during which sleep–wake cycles were monitored through actigraphic recording and sleep diaries. Sleep Efficiency Index (SEI), obtained as the ratio between total time spent sleeping divided by the total time spent in bed after lights off, was computed for each night. The SEIs were ordered from the worst to the best. Due to technical problems, two participants only had recording for 4 nights. Consequently, one night was excluded for the whole sample. Results showed that the SEI was always higher in good sleepers as opposed to people with insomnia which, moreover, presented higher within subjects and within group variability than did the control group. Regarding the physiological measures, it was found that sympathetic arousal measured by SCLs predicted the quality of the sleep during the subsequent night. Specifically, both groups showed marginally high rest arousal and significantly high arousal in response to stimulation (arousability) linked to low SEI during the subsequent night. The existence of a relationship between sympathetic arousal and sleep quality is consistent with previous results (e.g. Broman, Hetta, 1994). With respect to IBIs results evidenced no systematic change across both groups and nights of different qualities. In summary, results demonstrated that the variability of sleep quality, which is a central feature of insomnia, is also related to varying levels of evening autonomic arousal and arousability. The same relationship was found also in good sleepers.