Personality, mood, and psychophysiological correlates of Home and östberg's (1976) Morningness-Eveningness Questionnaire (MEQ) are reported. State data were collected at both morning and evening times of day. MEQ score was related to personality traits associated with psychopathology—trait anxiety in males and psychoticism in females. The best explanation of these correlations is that personality and subject sex affect attention to zeitgebers or time of day cues. MEQ score was related to extroversion only in female subjects. A between-subjects study showed that morningness was positively associated with high energetic arousal in the morning, as predicted, but the MEQ did not predict physiological arousal measures reliably. A subsequent within-subjects study demonstrated sex differences in interactive effects of time of day and morningness-eveningness in mood states related to psychological disorder. The MEQ predicted two somewhat independent circadian rhythms—one related to arousal, and one to depression. It is concluded that personality may be linked to morningness-eveningness through associations between personality traits and cognitive and social factors affecting the entrainment of circadian rhythms to the sleep-wake cycle.