A method of predicting substance levels from EEG data comprises the steps of analysing EEG data to obtain the average power for each of a plurality of predetermined frequency bands calculating an amalgamated value from the average powers derived for each frequency band said amalgamated value being calculated by combining the average powers for each frequency band by dividing and/or multiplying according to a predetermined order and obtaining an estimate of the hormone level from the equation Y=bX+C where Y is the substance to be predicted X is the amalgamated value and b and C are constants. In one embodiment the substance is Cortisol and the amalgamated value is the ratio Theta/Alpha/Delta/SMR.
