A method of predicting substance levels from EEG data is disclosed. The method includes analyzing EEG data to obtain the average power for each of a plurality of predetermined frequency bands and calculating a value from the average powers derived for each frequency band, said value being calculated by combining the average powers for each frequency band by dividing and/or multiplying according to a predetermined order. The method further includes obtaining an estimate of the hormone level from the equation Y=bX+C, where Y is the substance level to be predicted, X is the value and b and C are constants, wherein the substance is selected from hormones, neuro transmitters and bio markers.
