A sleep monitoring system (10) for monitoring the sleep of a pair of subject comprises a pair of C02 sensors (21,21′) for mounting in different sleeping regions within a space (1) and a processor (31) communicatively coupled to the pair of CO2 sensors. The processor is adapted to, for each CO2 sensor in a particular sleeping region, monitor (203) a CO2 concentration from sensor data produced by the CO2 sensor in said particular sleeping region to detect a presence of a subject in said sleeping region; to register (207) the monitored CO2 concentration upon detecting (205) said presence; to determine (303) a degree of crosstalk between said CO2 sensors upon detecting said presence; and to derive (305, 313) sleep pattern information for said subject from the registered CO2 concentration during said presence as a function of the determined degree of crosstalk, wherein the sleep pattern information comprises at least an indication that the subject is awake and an indication that the subject is asleep.