Spectral analysis of the sleep onset period in primary insomnia

•This study shows that the patients with primary sleep onset insomnia (SOI) and sleep maintenance insomnia (SMI) present different spectral characteristics during sleep onset period.•The lower level of beta (18–29.75 Hz) frequency band found in SOI in comparison with SMI suggests that the hyperarousal is not involved in the etiology of primary sleep onset insomnia.•The results of the current study seem to corroborate a “wake-sleep switch problem” as a basic mechanism of SOI.

ObjectiveTo compare the EEG power spectra characteristics of the sleep onset period (SOP) in patients with sleep onset insomnia (SOI), sleep maintenance insomnia (SMI) and good sleepers (GS).MethodsThe time course of EEG power density (1–40 Hz) during the SOP was examined in thirty subjects (SOI patients: N = 10, SMI patients: N = 10, GS: N = 10).ResultsThe EEG power of the beta2 frequency band (18–29.75 Hz) was significantly lower in SOI than in SMI in the period preceding sleep onset. The alpha power was significantly higher for the SMI group compared to GS before sleep onset. Despite the lack of statistical significance, several differences in EEG dynamics were observed in SOI compared to two other groups: delta power increased slower after sleep onset; beta2 and 3 (18–29.75 and 30–39.75 Hz) power decrease less abruptly before sleep onset; beta1 (15–17.75 Hz) power increase through the whole SOP.ConclusionsThe lower level of beta2 frequency band in SOI and the differences in dynamics in delta and beta bands may suggest that a mechanism other than hyperarousal participates in etiology of SOI.SignificanceSOI and SMI patients have different spectral characteristics in SOP, thus future studies should avoid the inclusion of mixed insomnia samples.