A system for determining the sleep state of a person provides indexes representing the variation of a biological signal such as pulse or respiration rate on the basis of first variation amount denoting a trend with time of the biological signal from the start of measurement and second variation amount denoting the temporal variation of the biological signal, and discriminates different sleep states on the basis of the distribution density of the variation indexes exceeding a predetermined threshold, whereby NREM and REM sleep periods in particular can be reliably discriminated. This provides sleep information which enables an awakening stimulus to be generated shortly after termination of REM sleep to ensure a comfortable awakening. <IMAGE>
System for discriminating sleep state