1. A method for assigning sleep stages using a reduced electrode setup wherein the patient is in an unattended sleep diagnostic scenario, the method comprising: acquiring electroencephalograph (EEG) data from two or more EEG electrodes of an electrode arrangement that is adapted to be attached on a frontal area of a patient's head; processing, at a processor, the EEG data into one or more epochs; recording, at the processor, the EEG data; analyzing, at the processor, the EEG data both in time domain and frequency domain so as to extract transients and rhythmic activity; calculating, at the processor, a first entropy time-series and a second entropy time-series from the EEG data for each said epoch, wherein the first entropy time-series characterizes states of light sleep and the second entropy time-series characterizes all sleep states including light sleep; calculating, at the processor, a differential entropy by subtracting the first entropy time-series from the second entropy time-series; assigning, at the processor, an epoch as light sleep when the absolute value of the differential entropy is minimum or deep sleep when the absolute value of the differential entropy is maximum; detecting, at the processor whether at least one graphoelement is present from the EEG data for each epoch; interpolating, at the processor, the differential entropy if there are no detected graphoelements, and if there are one or more detected graphoelements, then correlating the at least one graphoelement with the differential entropy; and assigning a particular sleep stage for the epoch when the one or more detected graphoelements and differential entropy are not contradictory to each other or assigning a particular sleep stage and not of any other sleep stage for the epoch when the one or more detected graphoelements and differential entropy are contradictory to each other.