Background: The bispectral index (BIS) is a parameter which reflects brain electrical activity (EEG) in a range of values between 0 au (minimum value, no EEG) and 100 au . Aim: We further assessed whether BIS can be useful for identifying sleep and the difference between its stages. Methods: The study was conducted in 14 patients being investigated for sleep apnea/hypopnea syndrome (OSA) . A full polysomnography through a night9s sleep was recorded and analyzed manually according to standard criteria for sleep staging. Throughout the process, from initial waking up to awakening, a continuous record of the BIS time-synchronized with the manual analysis of the PSG was performed. The average BIS value every minute was used as the dependent variable in the analysis. Results: Thirteen patients were diagnosed from OSA, with an apnea/hipoapnea index (AHI) of 53.6 ± 26.1. We analyzed a total of 5.579 minutes with synchronized measurements of BIS and PSG data. Overall, 91% of all EEG records corresponded to sleep period, representing 5.4% of stage 1, 68.9% of stage 2, 15.7% of stage 3-4 and 9.9% REM sleep. The mean BIS value was 88.9 ± 9.5 (mean ± SD) in wakeful state, 84.9 ± 12.4 in stage 1, 77.7 ± 11.4 in stage 2, 56.1 ± 13.4 in stages 3-4 and 76.8 ± 17.7 in REM sleep. Significant differences were observed between wakefulness and sleep in any stage (p ( 3-4 ) . Conclusions: BIS values decrease during sleep and change according to its stages. BIS values below 70 have a high sensitivity and specificity for detecting deep sleep stages.