The course of study of time-of-day effects on human performance has not been an easy one to chart, with many findings that seem to be in opposition. This review examines the difference between group and individual differences with regard to time-of-day effects; time-of-day effects in individuals; morningness-eveningness as an individual characteristic; morningness-eveningness in adolescents; effect of time of day on cognition and academic performance; time-of-day effects on intelligence, testing, and academic achievement; the effect of matching individuals to their preferred time on academic achievement; and motivation as a primary confounding variable in time-of-day preference/academic performance studies. Other possible confounding variables and procedures in testing time-of-day effects are also briefly examined.