The squirrel monkey (Saimiri sciureus) exhibits a robust daily rhythm of sleep-wakefulness that is under circadian control, but the nature of homeostatic sleep regulation in this diurnal primate is poorly understood. Since delta frequency (0.5-2.0 Hz) activity in the electroencephalogram (EEG) during non-Rapid Eye Movement (NREM) sleep is thought to reflect homeostatic factors contributing to sleep tendency, we measured EEG delta power density and slow wave incidence and amplitude during NREM sleep during spontaneous sleep, occurring when monkeys were housed undisturbed in a 24-hour light-dark (LD) cycle and in constant light (LL). In LD and LL conditions, monkeys exhibited circadian rhythms in delta power density, wave incidence and wave amplitude that peaked in the middle of the subjective night, several hours after consolidated sleep onset. These results differ from predictions of a purely homeostatic model of sleep that would include maximal levels of delta activity at sleep onset.