Physiologic indexes in chronic insomnia during a constant routine : Evidence for general hyperarousal?

Study Objectives: It has been hypothesized that general hyperarousal, present during both sleep and wakefulness, may underlie chronic insomnia. The present study explored, under strictly controlled conditions, whether chronic insomnia is associated with altered physiologic markers of arousal, both in absolute levels and in terms of circadian rhythmicity, relative to controls. Design: A 24-hour constant-routine protocol was implemented to assess physiologic measures. Setting: The study was conducted in an isolated, temperature- and light-controlled, sound-attenuated sleep laboratory. Participants: Eleven subjects with clinically diagnosed chronic insomnia were compared with 13 healthy matched controls. Interventions: The subjects underwent physiologic parameter recordings and cognitive performance testing during 24 hours of total sleep deprivation under strictly controlled circumstances. Measurements and Results: Cardiovascular parameters, free cortisol, and body temperature were subjected to mixed-model analysis of variance and mixed-model harmonic regression. Overall, no differences were found in either the absolute level or the circadian parameters (amplitude, phase) of these variables between the insomniacs and the control subjects. Conclusions: Although physiologic indexes of arousal were slightly elevated in the insomnia group relative to the controls, the differences between the groups were not statistically significant. This could have been due to a lack of statistical power or could reflect the actual absence of arousal in our sample of chronic insomniacs. Systematic interindividual level differences overwhelmed any differences between the 2 groups, making it unlikely that general hyperarousal was a critical underlying factor in our sample. Earlier findings of hyperarousal in insomnia during studies that allowed sleep may have been specifically related to the sleep state.