This study sought to determine whether the daily fluctuations in energy and tension; and Type-A personality affected aggression and mood. The study took place online during late morning and early evening. Eighty-one students were randomly assigned to one of those periods. Energy, tension levels, Type-A personality characteristics and mood were assessed. Participants wrote an essay, which received a scathing review by a confederate. Aggression against the reviewer was measured by the intensity and duration of white noise directed at the confederate. Results showed that participants during the early evening had more aggression and negative mood versus during the late morning. High Type-A participants exhibited higher levels of aggression during both times of the day than low Type-A participants. This study provides evidence of energy and tension shifts throughout the day and a Type-A personality trait as additional causes of aggressive behavior and negative mood. Directions for future research are discussed.