There is provided an apparatus for use in diagnosing the presence of obstructive sleep apnea in a patient, the apparatus comprising a processor configured to receive signals representing measurements of a patients breathing obtained during a plurality o breathing cycles by the patient while the patient is awake, convert the signals into the frequency domain and to determine a value for at least one parameter based on an analysis of the frequency-domain converted signals in one or more frequency bands covering frequencies below 100 Hz.
