Mental activity is a neglected factor in sleep research. The few investigations on sleep that manipulate prior mental activity are inconclusive with respect to the possible effects of mental activity on recovery. In the present study, the effects of two levels of mental activity on subsequent sleep were studied. Thirteen male subjects (range 18–28 years) participated in one lightly and two heavily mentally strenuous conditions in a counterbalanced order. Light mental activity included 8 h of relaxed video watching. The second condition consisted of performing computer tasks involving sustained attention, memory, logical thinking and calculations for eight consecutive hours. In the third condition, the same heavy mental workload was interspersed with breaks. Subjectively, the subjects rated the condition with heavy mental activity (without breaks) as mentally more strenuous than the condition with light mental activity. Subjects were significantly less awake shortly after sleep onset in the heavy-workload condition than in the light-workload condition. There were no differences between the conditions in any of the other visually scored sleep variables. The total amount of slow wave activity (SWA) and its discharge during the night was not affected by the level of mental activity or by the presence of breaks. These findings fail to support the proposition that SWA reflects a need for sleep that accumulates at a rate depending on mental activity during prior wakefulness.