Brain Research Bulletin 76 (2008) 85–89 Research report Wakefulness–sleep transition: Emerging electroencephalographic similarities with the rapid eye movement phase R´ obert B ´ odizs ∗ , Melinda Sverteczki, Eszter M´ esz´ aros Institute of Behavioural Sciences, Semmelweis University, Nagyv´ arad t´ er 4, H-1089 Budapest, Hungary Received 6 September 2007; received in revised form 9 November 2007; accepted 15 November 2007 Available online 18 December 2007 Abstract 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. © 2007 Elsevier Inc. All rights reserved. Keywords: Sleep stages; Polysomnography; Delta rhythm; Theta rhythm; Alpha rhythm; Hypnagogic hallucinations 1. Introduction Wakefulness, non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep are naturally occurring states of consciousness, differing substantially in chemical, electrical and metabolic brain processes, as well as in the characteristic subjective experiences accompanying them [9]. Although the core physiological features of these states could explain many of their phenomenological differences, the transition between wakefulness and sleep is still a kind of no mans land, as this poorly characterized period unifies some apparently unrelated features of wakefulness, NREM sleep and REM sleep. On the macrostructural level Stage 1 sleep is regarded as light NREM sleep [15]. Some quantitative EEG analyses revealed a con- tinuous increase in NREM sleep-specific activity during this sleep stage, thus suggesting its NREM sleep-like physiology [4]. However, at the subjective level the transition between wakeful- ∗ Corresponding author. Tel.: +36 1 210 2930x6404; fax: +36 1 210 2953. E-mail address: bodrob@net.sote.hu (R. B ´ odizs). ness and sleep is non-continuous, possessing transiently some characteristic features of REM sleep: i.e. vivid and bizarre hal- lucinatory activity [7], the dream-like quality of which depends on REM sleep pressure [13]. Moreover, frequent episodes of NREM sleep with low muscle tone were detected at sleep onset which may represent early manifestations of REM sleep [18]. However, a functional neuroimaging study concluded that the hypnagogic hallucinatory experience is in fact the dreaming state of wakefulness and not sleep [11]. Detailed EEG anal- yses of wakefulness–sleep transition (WST) revealed a quite complex picture consisting of nine stages with different electro- physiological features. Almost all of the nine stages could be accompanied by hypnagogic experiences, but the highest inci- dence was found in the fifth stage, characterized by theta activity appearing after alpha dropout/EEG flattening and before vertex sharp waves/sleep spindling [10]. The question arises if this tran- sitional period shares some EEG features of REM sleep and if yes, what are these features? The covert REM sleep hypothe- sis of dreaming suggests that hidden REM-like physiological elements appear in different parts of the sleep process determin- ing the concomitant increase in vivid, REM-type mental activity 0361-9230/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2007.11.013