Spectral power and fractal dimension: Methodological comparison in a sample of normal sleepers and chronic insomniacs

This methodological study compared the feasibility of the time domain fractal dimensions (FD) and a quantitative electroencephalogram (QEEG) when characterizing the daytime cortical activation in insomniacs and normal sleepers. We hypothesized that FD would yield additional non-redundant information when compared to conventional QEEG, suggesting that FD could be useful in future insomnia studies. The original source sample consisted of 157 adults (104 women and 53 men) aged 33–71 years. From this sample, 44 normal sleepers and 35 chronic habitual insomniacs were selected for this study. Conventional QEEG and time domain FD, calculated by the Higuchi algorithm, were obtained and analyzed on the day following a laboratory night. The reactivity of the cortical arousal caused by eye-closing and task performance-induced fatigue was operationalized in daytime electroencephalogram activity by FD and QEEG variables. FD, delta power, and beta power measures were independently able to predict chronic subjective-type insomnia in multivariable binary logistic regression models. The cortical arousal reactivity was not related to the previous night’s subjective sleep quality. The hypothesis that FD may yield additional non-redundant information about cortical arousal in insomniacs when compared to information carried by conventional QEEG was supported. The findings provide cumulative support for the hyperarousal theory of insomnia. No support, however, was gained for the hypothesis that the cortical hyperarousal, as defined by FD, would be modulated by the previous night’s perceived quality of sleep. In combination with conventional QEEG measures, FD might be used to refine the descriptions of insomnia subtypes.