Measurements of physiological, biochemical and psychological variables at two or more different times of day reveal substantial inter-individual differences. This paper reviews studies which have dealt with these differences in terms of the morningness-eveningness, personality (introversion-extraversion), age or sex of their subjects. Studies of individual differences in the response of the circadian system to disturbance (e.g. shift work) are also discussed. The most reliable differences were observed in association with the morningness-eveningness factor. From the studies reviewed here it appeared that several rhythm parameters covaried consistently as a function of morningness-eveningness, suggesting underlying differences in the intrinsic period of the circadian system. It is argued that the differences in rhythm parameters associated with the personality dimension of introversion-extraversion are the result of exogenous influences. The results with regard to age-related and sex-related differences were not sufficient to allow conclusions to be made.