A computer-implemented method for determining a sleep state of a user, the method comprising: receiving a physiological signal from a physiological signal detector used by the user, wherein the physiological signal comprises data from physiological measurements including an electrocardiogram, a polysomnogram, eye movements, muscle activity and/or heart rate; determining, based on the received physiological signal, the sleep state of the user; and calculating a first reliability value associated with the determination, the first reliability value being calculated based on a comparison of the received physiological signal with historic physiological signals of the same sleep state as the determined sleep state, characterised in that the method further comprises: when the first reliability value is lower than a second threshold value: requesting data from a peripheral device; determining a level of correlation between the data from the peripheral device and the received physiological signal; calculating a second reliability value associated with the determination of the sleep state of the user, the second reliability value based on the level of correlation; and updating the first reliability value if, based on the first reliability value and the second reliability value, the determined sleep state is more reliably determined, wherein peripheral devices include any of an inertia sensor, a temperature sensor, a strain sensor and a skin conductance sensor. receiving a physiological signal from a physiological signal detector used by the user, wherein the physiological signal comprises data from physiological measurements including an electrocardiogram, a polysomnogram, eye movements, muscle activity and/or heart rate; determining, based on the received physiological signal, the sleep state of the user; and calculating a first reliability value associated with the determination, the first reliability value being calculated based on a comparison of the received physiological signal with historic physiological signals of the same sleep state as the determined sleep state, characterised in that the method further comprises: when the first reliability value is lower than a second threshold value: requesting data from a peripheral device; determining a level of correlation between the data from the peripheral device and the received physiological signal; calculating a second reliability value associated with the determination of the sleep state of the user, the second reliability value based on the level of correlation; and updating the first reliability value if, based on the first reliability value and the second reliability value, the determined sleep state is more reliably determined, wherein peripheral devices include any of an inertia sensor, a temperature sensor, a strain sensor and a skin conductance sensor. requesting data from a peripheral device; determining a level of correlation between the data from the peripheral device and the received physiological signal; calculating a second reliability value associated with the determination of the sleep state of the user, the second reliability value based on the level of correlation; and updating the first reliability value if, based on the first reliability value and the second reliability value, the determined sleep state is more reliably determined, wherein peripheral devices include any of an inertia sensor, a temperature sensor, a strain sensor and a skin conductance sensor.