Inter-individual differences in the human circadian system: A review.

Reviews studies that have dealt with interindividual differences revealed by measurements of physiological, biochemical, and psychological variables at 2 or more times of day in terms of the morningness-eveningness, personality (introversion-extraversion [I-E]), age, or sex of their Ss. 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, it is concluded 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 I-E personality dimension are the result of exogenous influences. The results with regard to age- and sex-related differences were viewed as insufficient to allow conclusions to be made. A table that summarizes studies on I-E differences as a function of time of day is presented. (6 p ref) (PsycINFO Database Record (c) 2000 APA, all rights reserved)(unassigned)