Sixty-one men and 50 women measured their own skin conductance hourly throughout one working day as well as recording drug intake and activities; they also completed the EPQ, I7 and Morningness-Eveningness questionnaires. As expected, self-reported Morning types showed higher skin conductance in the morning and Evening types in the evening. Introverts appeared as more highly aroused than extroverts, but particularly so in the morning; by midnight the two personality types had converged in skin conductance. Sociability, rather than impulsiveness, seemed to be the component of extraversion responsible for this interaction. Women showed higher conductance in the early morning, men in the afternoon and evening. Indications were that activity was determining arousal rather than the other way about, and that drug use was socially rather than neurologically motivated. © 1990.