Sleep state misperception: Is it a true subjective insomnia?

Introduction. Sleep state misperception (SSM) is a subtype of primary insomnia (PI) characterized by normal conventional PSG measures in patients having profound and, at times, dramatic sleep complaints. Although this PI subtype has been included into classification systems about 20 years ago (1), little research has been devoted to this subtype (2), and only one study has assessed quantitative EEG changes showing lower delta and greater alpha, sigma, and beta EEG activity in NREM, but not in REM sleep (3). The aim of the present study was to assess, for the first time, the EEG correlates of SSM by means of two separate Hz-by-Hz EEG power analyses of the sleep-wake transition and of the whole night. Moreover, the presence of regionally-graded EEG differences along the antero-posterior axis have been also evaluated. Methods. Standard EEG recordings from four midline derivations (Fz-A1, Cz-A1, Pz-A1, Oz-A1) of 10 patients with a diagnosis of SSM (age=35.6 ±13.9yrs) were compared to those of 10 normal sleepers. Power spectra of the four derivations were computed by a Fast Fourier Transform routine for a final 1-Hz resolution (1-30 Hz range). Power spectra were calculated for the presleep and the sleep onset periods (5 min samples), and for the whole night, separately for NREM (S2 + SWS) and REM sleep. Results. The two groups showed significant differences in the presleep period only at the occipital site, with higher alpha relative power in normal than in SSM subjects. After sleep onset, a distinct pattern of topographical changes differentiated the two groups: SSM patients showed lower delta relative power over anterior areas (Fz and Cz) and higher beta activity (more pronounced at anterior sites) as compared to normals. NREM sleep of the whole night again showed lower delta and higher beta activity in SSM patients than in normals. A clear dissociation between anterior and posterior areas was found with the SSM patients showing higher beta at the anterior sites and lower beta at Oz as compared to normals. Analyses on REM sleep also confirmed this antero-posterior dissociation for the beta relative power. Conclusions. Although preliminary, these results a) point to the existence of speciphic physiological correlates of SSM, confuting its definition as subjective insomnia; b) explain the understimation of sleep latency and of total time in terms of hyperarousal and heightened awareness during the sleep onset process, and continuing across the whole night; c) localize this mechanism, expressed by lower delta and higher beta EEG activity, at the more anterior areas, mainly at the frontal cortex.