NUMEROUS methods have been devised to measure the depth of sleep.1 Before the development of electroencephalography, body motility and the auditory arousal threshold were widely studied as indices of sleep depth in man. It was found that body motility increased during the latter part of a night's sleep,1,2 while the auditory arousal threshold fell during the same period.1 Thus, it appeared that sleep became progressively more shallow toward the end of the night. Mullin et al3 found that the auditory arousal threshold was positively correlated with the length of time that had elapsed since the subject's last body movement, with more frequent movements and greater ease of arousal during the last few hours of the night. They postulated a cyclic pattern of sleep depth, with the cycles becoming more shallow toward the end of the night, to replace the simpler concept of rapid onset
Pollak CP, Weitzman ED, Kripke DF. Arousal Threshold Ranges: Threshold Ranges as Determined by Electrical Stimulation of the Brain During Stages of Sleep in the Monkey (Macaca mulatta). Arch Neurol. 1967;17(1):94–102. doi:10.1001/archneur.1967.00470250098011
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