Morningness (a preference for earlier rise and bed times) is associated with well-being, better sleep quality, and positive affect, while eveningness is associated with poor sleep quality, negative mood, maladaptive behaviours, and psychological disorder. The current study investigated: (1) whether eveningness is associated with more maladaptive metacognitive beliefs; (2) whether maladaptive metacognitive beliefs and sleep quality are related to associations between morningness–eveningness and affect. An online survey with questionnaire measures of morningness–eveningness, affect, sleep quality, and metacognitive beliefs was completed by 591 undergraduate students. More morningness correlated with more positive affect, while more eveningness correlated with more negative affect and poor sleep quality. Eveningness also showed small correlations with having less cognitive confidence and with metacognitive beliefs about uncontrollable thoughts. Mediation analysis showed that cognitive confidence and beliefs about uncontrollable thoughts, together with poor sleep quality (especially subjective sleep quality and daytime dysfunction), were mediators in the relationships between morningness–eveningness and both negative affect and positive affect. These findings suggest that metacognitive beliefs may be an important consideration in understanding the associations between morningness–eveningness and well-being.