SUMMARY. The aim of this study was to examine whether BIS values during sleep correspond to the different sleep stages, in order to assess BIS as an alternative means of sleep staging. Patients-Methods: The study was conducted on 23 patients who were examined concurrently with polysomnography (PSG) for diagnosing sleep-disordered breathing and with BIS. Exclusion criteria were sleep duration <4 hours, sleep efficiency <80% on PSG and signal quality index (SQI) <50% on BIS. Comparisons in recordings were performed. Results: The patients provided 806 different sleep periods. The mean BIS value was 93.6±4.8 in the wakeful state, and in sleep, according to each stage: 84±11.5 in stage 1, 75.4±13.2 in stage 2, 53.4±15.8 in slow wave sleep (SWS), and 81.5±13.3 during REM sleep. A significant difference was observed between BIS values in the wakeful state and stage 1 (p<0.005) and between stages 1 and 2 and SWS (p<0.001), but not between stage 1 and REM (p=0.102). Conclusion: BIS values decrease with sleep and remain low, with the exception of REM sleep, the BIS values in which overlap with those in stage 1, reducing the sensitivity of BIS in sleep staging. Pneumon 2009, 22(3):235-239.