Summary Insomniacs report daytime functioning problems, but studies of neurobehavioral functioning in insomniacs have shown little objective evidence of impairment. In addition, very little is known about the influence of the circadian clock on performance in chronic insomniacs. In the present study, we investigated whether chronic insomnia is associated with an overall performance deficit, and what the effect is of circadian rhythmicity, under strictly controlled laboratory conditions. A 24-h experiment was carried out under constant routine conditions. Psychomotor performance, body temperature, and subjective functioning of 11 insomniacs and 13 healthy subjects were assessed. The insomniacs showed significant overall performance impairments in vigilance, working memory, and motor control. In addition, body temperature, performance and subjective functioning showed a circadian pattern similar to healthy subjects, with trough values in the late night/early morning and peak values in the early evening. Self-reported functioning among the insomniacs indicated mood disturbances, concentration problems, elevated fatigue and elevated sleepiness. The results indicated that chronic insomnia is associated with a substantial lowering of the 24-h level of performance and subjective functioning, irrespective of the type of task and/or the particular parameter, and without differential effects of circadian rhythmicity. Apparently, chronic insomnia has a negative impact upon performance as measured under strictly controlled, unmasked conditions.