Mood and affect are often assessed in psychological research without regard for the potential influence of time of day or other environmental factors. This study examined the determinants of daily mood variability. Ninety-four subjects completed a diary every 15 min for 1 day, and subjects' moods, activities, and locations were assessed. Diurnal cycles were found for several moods (e.g., happy, tired), activities (e.g., discourse, thinking), and locations (e.g., home, work). Diurnal cycles in most moods were strongly associated with activities and location, yet diurnal cycles of some moods (rushed, sad, and tired) were not dependent on these factors. These findings have implications for mood assessment: Mood assessment performed only at particular times of day, or those relying on point assessment, may be influenced by time of day and by the activities and location of the subject.