THE RELATIONSHIP BETWEEN MORNINGNESS-EVENINGNESS, PERSONALITY AND HABITUAL CAFFEINE CONSUMPTION

Individual differences in circadian rhythms have been found to be related to personality variables, including extraversion and psychoticism, and to the consumption of stimulant drugs like caffeine. This study investigated differences in both personality variables and caffeine consumption in Morning-type (M-type) and Evening-type (E-type) subjects. The Eysenck Personality Questionnaire (EPQ) and a caffeine consumption questionnaire were administered to 47 M-type and 69 E-type individuals, classified according to the Home-Ostberg Morningness-Eveningness Questionnaire (MEQ). E-type individuals scored significantly higher than M-types on both the extraversion and psychoticism dimensions of the EPQ, while no difference was found on neuroticism scores. E-types also consumed significantly more caffeine than M-type individuals. The possibility that the personality variables of extraversion and psychoticism lead particular individuals to consume higher levels of caffeine and to prefer a night-active life-style is discussed. However, an alternative explanation for the interaction between caffeine use, individual differences in circadian rhythms, and personality, is proposed: namely, that caffeine consumption (which may be mediated by underlying differences in basal arousal levels, as reflected by personality variables) may exert a direct phase-delaying effect on the circadian system.