An active medical device having an improved discrimination between an awake phase of a patient and a sleep phase of a patient. This device measures a physiological parameter of a patient, delivers a physiological signal with a slow time response variation, in particular a signal of minute-ventilation (signal MV), and detects whether the patient is in a phase of awakening or sleep. An average (VE) of the signal MV is calculated over a given number of respiratory cycle and compared with a predetermined threshold (Threshold MV), such that a first state of awakening is determined when the average is higher than the threshold, and a first state of sleep is determined in the contrary case. The device also measures patient activity (signal G), using a signal having a short response time. The activity signal is compared with a predetermined threshold (Threshold G), and a second state of the patient is determined to be an awakening phase when the activity signal is higher than the threshold, and a sleep state in the contrary case. Detecting the phases of awakening and sleep also includes comparing the first state of awakening/sleep to the second state of awakening/sleep, and in the event of a discordance between these states, the response time of the physiological signal processing is modified, in particular by reducing the number of respiratory cycles over which the average MV signal (VE.sub.128, VE.sub.64) is calculated.