Abstract: A simple system for determining a non-invasive sleep state of a person is described. The system comprises: (a) means for measuring the person's cardiac R-R waves interval; (b) means for calculating the power spectrum of the cardiac R-R interval, thus obtaining a ratio between spectral power in the and high frequencies; (c) means responsive to the measuring and calculating means for generating output signals, one output signal having a first value when said ratio is above a specific predetermined threshold, designated as REM sleep-and one outputs signal having a second value when the above ratio is below this threshold, designated as non-REM sleep, and (d) means responsive to the output signal for designating a time period of predetermined duration as a REM period or a non-REM period, respectively. The system enables an accurate determination of the sleep state.