A sleep stage classification system (330) is provided. A sleep stage analyzing is performed based on an output signal (100a) of a PPG sensor (100). An estimation unit (310) is adapted to estimate features associated to a respiratory effort of a user based on the output signal (100a) of the PPG sensor (100) and to output a respiratory effort feature signal. A respiratory effort determining unit (320) is adapted to determine to respiratory effort based on the respiratory effort feature signal from the estimation unit (310). A sleep stage classification unit (330) is adapted to classify a sleep stage of a user based on the determined respiratory effort from the respiratory effort determining unit (320) and to output a sleep stage classification signal (310).
