Insomnia Patients with Insomnia Often Report Awareness of Environmental Activities for an Extended Period of Time While Lying in Bed Trying to Fall

PATIENTS WITH INSOMNIA OFTEN REPORT AWARENESS OF ENVIRONMENTAL ACTIVITIES FOR AN EXTENDED PERIOD OF TIME WHILE LYING IN BED TRYING TO FALL asleep.1-3 They also tend to report that they are still awake even though polysomnographic recording indicates sound sleep status.4 Therefore, patients who complain of insomnia often overestimate their sleep-onset latency and underestimate theirtotal sleep time.5-7 Perlis and his colleagues have proposed a hyperarousal hypothesis for insomnia from a neurocognitive perspective that offers a possible explanation of the above phenomenon.8 This model hypothesizes that the difficulties in sleep initiation and/ or sleep maintenance in patients with primary insomnia may be associated with increased information processing around the onset of sleep. Accordingly, patients with insomnia may have an elevated degree of cortical or cognitive arousal that corresponds to enhanced information processing during polysomnographydefined sleep. Thus, they tend to perceive wakefulness even though the electroencephalogram (EEG) indicates sleep. This point of view is primarily supported by the demonstration that high-frequency EEG power (beta range: 14-35 Hz; gamma range: 35-45 Hz) prior to and during sleep are elevated in patients with insomnia, as compared with normal controls.9-12 Furthermore, their beta power during sleep correlates with the discrepancies between subjective and polysomnography-defined sleep.11 After treatment with cognitive behavioral therapy, beta activities have been found to decrease significantly.13,14 Because high-frequency EEG has been reported to be associated with increased cognitive activities, these findings support the hypothesis that information processing during sleep is increased in patients with insomnia. Although-high frequency EEG is usually thought to be associated with higher mental activities, there are alternative explanations of these findings. Bonnet and Arand have pointed out that increased muscle tension can also contribute to highfrequency EEG.15 They have shown that high-frequency EEG power can be elevated simply by having the subjects engage in some mild physical activities, such as standing up and sitting down or walking around. Therefore, the elevated high-frequency EEG observed in insomniacs is not necessarily associated with increased cortical activities. In addition, even if the highfrequency EEG reflects increased mental activities in insomniacs, it is not necessarily associated with enhancement of processing of environmental stimuli. High-frequency EEG, in fact, could reflect heightened anxiety, rumination, or both. The present study was undertaken to further evaluate information processing during sleep in patients with insomnia with the recording of event-related potentials (ERPs). ERPs can reflect the neurophysiologic activities elicited by sensory stimulation and do not require behavioral responses or conscious awareness. It is therefore an ideal technique to study information processing during sleep. Previous studies on the ERP changes during sleep have shown an attenuation of the N1 and an enhancement of the P2 (also called P220) as a person falls asleep.16,17 Also, several ERP components have been reported to appear during non-rapid eye movement (NREM) sleep, including N350, P450, N550, and P900.18-20 The N1 has been suggested to be associated with the formation of auditory feature traces and an automatic switching of attention toward novel stimuli, whereas the P2 has been related to inhibition of sensory interferences. The attenuated N1 and enhanced P2 reflect decreased cortical excitability and ERP Evidence of Enhanced Excitatory and Reduced Inhibitory Processes of Auditory Stimuli During Sleep in Patients With Primary Insomnia