 A first reason for thinking that dreams are experiences during sleep is the relationship between dreaming and REM (rapid eye movement) sleep. Researchers in the 1950s discovered that sleep is not a uniform state of rest and passivity, but there is a sleep architecture involving different stages of sleep that is relatively stable both within and across individuals (Aserinsky &amp; Kleitman 1953, 1955; Dement &amp; Kleitman 1957). Following sleep onset, periods of non-REM (or NREM) sleep including slow wave sleep (so called because of the presence of characteristic slow-wave, high-voltage EEG activity) are followed by periods of high-frequency, low-voltage activity during REM sleep. EEG measures from REM sleep strongly resemble waking EEG. REM sleep is additionally characterized by rapid eye movements and a near-complete loss of muscle tone (Dement 1999: 27–50; Jouvet 1999).