OBJECTİVE: To investigate the bispectral index values of normal sleep stages PATİENTS and METHOD: Simultaneous polysomnography and bispectral index recordings of 21 patients who underwent polysomnography for diagnosis of obstructive sleep apnea were analyzed for determining the BIS values of normal sleep stages. RESULTS: BIS values are found sensitive in reflecting the various stages of natural sleep except for REM and stage N2. REM and stage N2 BIS values were close to each other. CONCLUSION: BIS is sensitive for the evaluation of the stages of natural sleep and BIS values decreases as the sleep stages becomes deeper