Apart from differences in circadian phase position, individuals with different morningness-eveningness levels vary in many more characteristics. Particularly consistent relationships have been observed between morningness-eveningness and mood. Eveningness has been associated with disadvantageous mood, e.g. depressiveness in healthy individuals, and mood disorders. A concept of social jetlag suggests that evening subjects function in less advantageous environments due to discrepancies between internal and social time (societies promote morning-oriented functioning), which results in their lowered mood. Individual temperament, as defined by the Regulative Theory of Temperament (RTT), refers to the capacity of the human organism to meet environmental requirements - the greater the capacity, the less negative impact of external conditions. Thus, the aim of this study is to determine which RTT traits are linked to both morningness-eveningness and mood dimensions and to test whether they account for the relationship between morningness-eveningness and mood. A sample of 386 university students (267 female) aged between 19 and 47 (M = 21.15, SD = 4.23) years completed the University of Wales Institute of Science and Technology (UWIST) Mood Adjective Check List, Morningness-Eveningness Questionnaire and Formal Characteristics of Behaviour - Temperament Inventory. Analyses revealed lower endurance (EN) and higher emotional reactivity (ER) related to eveningness as well as to lower hedonic tone (HT), energetic arousal (EA) and to higher tense arousal (TA). Moreover, eveningness was associated with lower HT, EA and higher TA. Among RTT traits, EN was most strongly related to eveningness, and mediation analyses revealed that this temperamental trait fully mediated the relationship between eveningness and the three mood dimensions. The remaining RTT traits did not provide more explanation of the association between morningness-eveningness and mood than EN itself. If subjects did not differ in EN, the association between morningness-eveningness and mood was absent. EN is discussed as a protective factor against negative consequences of social jetlag and particularly lowered mood in evening individuals.