Sleep and mental health can affect cognition and academic performance. The present study aimed to investigate the relationships between sleep, mental health, working memory, and academic performance. We collected demographic data from university students during the non-academic summer period and the academic term. We also measured academic performance (GPA), sleep (PSQI), depression (PHQ-9), anxiety (GAD-7), and disordered social media use (SMDS). Working memory was assessed by the Cambridge Neuropsychological Test Automated Battery (CANTAB). We assessed 83 students (42.2% male) with a mean age of 21 years. Compared to the non-academic summer period, students had significantly worse sleep and distress scores in the academic term. Anxiety, depression, and distress scores were significantly correlated with worse sleep quality. Despite worse mental health and sleep in the academic term, working memory improved compared to the non-academic summer period and was also correlated with a higher GPA. However, a higher GPA was significantly associated with longer sleep latency, increased sleep disturbances, and increased use of sleep medication. Students experiencing poor sleep suffered from poor mental health, although they maintained high GPA and working memory scores. Cognitive resilience, including higher working memory, may mask poor sleep quality and mental health among university students.