The invention relates to a method and device for contactless assessment of fatigue of a user, in particular a vehicle operator. The invention is comprised of a seamless continuous evaluation of cardiac activity variability and uses this data as an input for evaluation and quantitative assessment of sympathetic and parasympathetic activity of the autonomic nervous system, which are reliable markers of the physiological condition of a user. The small device is fixed to a driver's safety belt or embedded in the back of driver's seat without direct contacts to a user's body. The device continuously monitors the fatigue level of a user and alerts the user if he/she unexpectedly drifts from wakefulness to drowsiness. The invention significantly improves the safety of a vehicle operator.