Wakefulness–sleep transition: Emerging electroencephalographic similarities with the rapid eye movement phase

The covert-rapid eye movement (REM) sleep hypothesis of dreaming suggests that elements of REM sleep emerge during sleep onset, leading to vivid hypnagogic imagery. We tested the physiological part of this hypothesis by analysing scalp-recorded electroencephalograms of 15 human subjects during wake–sleep transition and subsequent night time sleep. Wake–sleep transition was categorised semi-automatically as alpha activity, alpha dropout and as early Stage 2 sleep. The slow oscillation, the slow and the fast subdivisions of the delta and the theta frequencies respectively, as well as alpha and sigma bands were analysed. The similarity of individual-specific wake–sleep transition periods and the whole night Stage 2 or REM sleep periods was expressed in a composite similarity measure covering the spectral power of all analysed frequency bands and in frequency-specific similarities related to power values in single bands. A significant increase in composite similarity with the whole night REM sleep emerged in the period of alpha dropout and diminished in early Stage 2 sleep. The alpha dropout period was more similar to whole night REM sleep than to whole night Stage 2 sleep. These region-independent effects were mirrored in region-specific manner by frequency bands of the delta-slow theta range. Findings are in accordance with the covert REM sleep hypothesis, with previous electrocorticographic results and with the frequency range of the sawtooth waves in humans.