A syndrome sometimes associated with sleeping late is identified in a student population. It is recognized as feelings of being worn-out, tired, lethargic, and irritable, with fuzzy thinking and difficulty in getting going. When it occurs, it tends to last for an average of 4.4 hr. This syndrome is particularly marked following sleep of 10 or more hr when the individual is not making up a sleep deficit. Under these conditions, the worn-out syndrome is more typical than feeling just great. These data suggest the testable hypothesis that optimal durations of sleep fall into a relatively narrow band and that disruptive psycho-physiological effects are sustained with both too much and too little sleep.