Disturbed intracortical excitability is a stable trait of chronic insomnia

Chronic insomnia is a poorly understood disorder. Risk factors for developing chronic insomnia are largely unknown, yet disturbances in arousal seem to accompany the disorder. We here investigate whether insomnia patients and control participants differ with respect to one operationalization of arousal, i.e. cortical excitability. A method to directly investigate the excitability level of the human cerebral cortex is offered by transcranial magnetic stimulation (TMS). We used singleand double-pulse TMS to investigate cortical excitability in 16 insomnia patients and 14 carefully matched control participants. Non-pharmacological insomnia patients showed, first, an exaggerated absolute response to both suprathreshold singleand double-pulse stimulation compared to control participants; secondly, a reduced relative response to double-pulse stimulation at long inter-pulse intervals. The abnormal excitability persisted despite sleep therapy that effectively improved sleep quality as well as behavioral and neuroimaging indices of brain function. The results suggest that a subtly disturbed intracortical excitability characterizes patients with chronic insomnia: a relatively reduced intracortical facilitation in the context of a globally increased absolute excitability. The findings do not resemble TMS-findings after sleep deprivation or in sleep apnea and thus seem specific to insomnia. They may offer diagnostic value and implications for assessment of risk to develop this common and disabling disorder. Disturbed intracortical excitability is a stable trait of chronic insomnia | 81