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How does physiological sleep affect the Bispectral Index (BIS)? We collected electroencephalographic (EEG) data from five subjects during the early part of the night, comparing the changes in the BIS with the conventional EEG stages of sleep. We found that the BIS was a consistent marker of depth of sleep. Light sleep occurred at BIS values of 75-90, slow-wave sleep occurred at BIS values of 20-70, and rapid eye movement sleep occurred at BIS values of 75-92. The effects of natural sleep on the BIS seem to be similar to the effects of general anesthesia on the BIS. The BIS may have a role in monitoring depth of sleep. Implications: Electroencephalographic data were collected from five subjects during sleep. We found that the Bispectral Index decreased during increasing depth of sleep in a fashion very similar to the decrease in Bispectral Index that occurs during general anesthesia. This study further highlights …