The academic performance of 104 fourth-year medical students was assessed in relation to their sleep habits reported in a questionary and their scores on the Minnesota Multiphasic Personality Inventory. Poorer academic performance was related significantly to later times of waking up in the morning, particularly at weekends, and to subjectively poorer quality sleep, but not to the amount of sleep usually obtained. Poor academic performance was related also to scores on scales 3 (hysteria), 4 (psychopathic deviate) and 8 (schizophrenia) of the MMPI. Simple enquiries about sleep habits may make it easier for students who are at greatest risk of academic failure to be identified and helped.