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Physiological studies have tended to concentrate on hypnagogia in the strict sense of spontaneous sleep onset experiences. Such experiences are associated especially with stage 1 of [[NREM sleep]],[Rechtschaffen, A., & Kales, A. (1968). ''A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects''. Washington, DC: Public Health Service, U.S. Government Printing.] but may also occur with pre-sleep [[alpha wave]]s.[{{cite journal | author = Foulkes D., Vogel G. | year = 1965 | title = Mental activity at sleep onset | journal = Journal of Abnormal Psychology | volume = 70 | issue = 4 | pages = 231–43 | doi=10.1037/h0022217| pmid = 14341704 }}][{{cite journal | author = Foulkes D., Schmidt M. | year = 1983 | title = Temporal sequence and unit composition in dream reports from different stages of sleep | journal = Sleep | volume = 6 | issue = 3 | pages = 265–80 | doi = 10.1093/sleep/6.3.265 | pmid = 6622882 | doi-access = free }}] Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha [[EEG]] activity.[{{cite journal | vauthors = Nielsen T, Germain A, Ouellet L | year = 1995 | title = Atonia-signalled hypnagogic imagery: Comparative EEG mapping of sleep onset transitions, REM sleep, and wakefulness | journal = Sleep Research | volume = 24 | page = 133 }}] |
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Physiological studies have tended to concentrate on hypnagogia in the strict sense of spontaneous sleep onset experiences. Such experiences are associated especially with stage 1 of [[NREM sleep]],[Rechtschaffen, A., & Kales, A. (1968). ''A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects''. Washington, DC: Public Health Service, U.S. Government Printing.] but may also occur with pre-sleep [[alpha wave]]s.[{{cite journal | author = Foulkes D., Vogel G. | year = 1965 | title = Mental activity at sleep onset | journal = Journal of Abnormal Psychology | volume = 70 | issue = 4 | pages = 231–43 | doi=10.1037/h0022217| pmid = 14341704 }}][{{cite journal | author = Foulkes D., Schmidt M. | year = 1983 | title = Temporal sequence and unit composition in dream reports from different stages of sleep | journal = Sleep | volume = 6 | issue = 3 | pages = 265–80 | doi = 10.1093/sleep/6.3.265 | pmid = 6622882 | doi-access = free }}] Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha [[EEG]] activity.[{{cite journal | vauthors = Nielsen T, Germain A, Ouellet L | year = 1995 | title = Atonia-signalled hypnagogic imagery: Comparative EEG mapping of sleep onset transitions, REM sleep, and wakefulness | journal = Sleep Research | volume = 24 | page = 133 }}] |
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To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of [[alpha wave|alpha]] (stages 1–3), suppressed waves of less than 20μV (stage 4), [[theta wave|theta]] ripples (stage 5), proportions of [[sawtooth wave|sawtooth]] waves (stages 6–7), and presence of [[sleep spindle|spindles]] (stages 8–9).[Hori, T., Hayashi, M., & Morikawa, T. (1993). Topographical EEG changes and hypnagogic experience. In: Ogilvie, R.D., & Harsh, J.R. (Eds.) ''Sleep Onset: Normal and Abnormal Processes'', pp. 237–253.] Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples).[{{cite journal | author = Germaine A., Nielsen T.A. | year = 1997 | title = Distribution of spontaneous hypnagogic images across Hori's EEG stages of sleep onset | journal = Sleep Research | volume = 26 | page = 243 }}] |
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To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of [[alpha wave|alpha]] (stages 1–3), suppressed waves of less than 20μV (stage 4), [[theta wave|theta]] ripples (stage 5), proportions of [[sawtooth wave|sawtooth waves]] (stages 6–7), and presence of [[sleep spindle|spindles]] (stages 8–9).[Hori, T., Hayashi, M., & Morikawa, T. (1993). Topographical EEG changes and hypnagogic experience. In: Ogilvie, R.D., & Harsh, J.R. (Eds.) ''Sleep Onset: Normal and Abnormal Processes'', pp. 237–253.] Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples).[{{cite journal | author = Germaine A., Nielsen T.A. | year = 1997 | title = Distribution of spontaneous hypnagogic images across Hori's EEG stages of sleep onset | journal = Sleep Research | volume = 26 | page = 243 }}] |
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The "covert-rapid-eye-movement" hypothesis proposes that hidden elements of [[REM sleep]] emerge during the wakefulness-sleep transition stage.[{{cite journal|last1=Bódizs|first1=Róbert|last2=Sverteczki|first2=Melinda|last3=Lázár|first3=Alpár Sándor|last4=Halász|first4=Péter|title=Human parahippocampal activity: non-REM and REM elements in wake–sleep transition|journal=Brain Research Bulletin|volume=65|issue=2|year=2005|pages=169–176|issn=0361-9230|doi=10.1016/j.brainresbull.2005.01.002|pmid=15763184|s2cid=20787494|doi-access=free}}] |
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The "covert-rapid-eye-movement" hypothesis proposes that hidden elements of [[REM sleep]] emerge during the wakefulness-sleep transition stage.[{{cite journal|last1=Bódizs|first1=Róbert|last2=Sverteczki|first2=Melinda|last3=Lázár|first3=Alpár Sándor|last4=Halász|first4=Péter|title=Human parahippocampal activity: non-REM and REM elements in wake–sleep transition|journal=Brain Research Bulletin|volume=65|issue=2|year=2005|pages=169–176|issn=0361-9230|doi=10.1016/j.brainresbull.2005.01.002|pmid=15763184|s2cid=20787494|doi-access=free}}] |
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