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Shortly after Freud's death, the study of dreaming from the perspective of neuroscience began in earnest. Initially, these studies yielded results which were hard to reconcile with the psychological conclusions set out in Freud's The Interpretation of Dreams (see footnote 1). The first major breakthrough came in 1953, when Aserinsky and Kleitman discovered a physiological state which occurs periodically (in 90 minute cycles) throughout sleep, and occupies approximately 25% of our sleeping hours. This state is characterized, amongst other things, by heightened brain activation, bursts of rapid eye movement (REM), increased breathing and heart rate, genital engorgement and paralysis of bodily movement. It consists, in short, in a paradoxical physiological condition in which one is simultaneously highly aroused and yet fast asleep. Not surprisingly, Aserinsky and Kleitman suspected that this REM state (as it came to be known) was the external manifestation of the subjective dream state. That suspicion was soon confirmed experimentally by Aserinsky and Kleitman and Dement and Kleitman (1957a, 1957b). It is now generally accepted that, if someone is awakened from REM sleep and asked whether or not they have been dreaming, they will report that they were dreaming in as many as 95 % of such awakenings. Non-REM sleep, by contrast, yields equivalent dream reports at a rate of only 5-10% of awakenings.