Individual differences in vulnerability to neurobehavioral performance impairment during sleep deprivation are considerable and represent a neurobiological trait. Genetic polymorphisms reported to be predictors have suggested the involvement of the homeostatic and circadian processes of sleep regulation in determining this trait. We applied mathematical and statistical modeling of these two processes to psychomotor vigilance performance and sleep physiological data from a laboratory study of repeated exposure to 36h of total sleep deprivation in 9 healthy young adults. This served to quantify the respective contributions of individual differences in the two processes to the magnitudes of participants’ individual vulnerabilities to sleep deprivation. For the homeostatic process, the standard deviation for individual differences was found to be about 60% as expressed relative to its group-average contribution to neurobehavioral performance impairment. The same was found for the circadian process. Across the span of the total sleep deprivation period, the group-average effect of the homeostatic process was twice as big as that of the circadian process. In absolute terms, therefore, the impact of the individual differences in the homeostatic process was twice as large as the impact of the individual differences in the circadian process in this study. These modeling results indicated that individualized applications of mathematical models predicting performance on the basis of a homeostatic and a circadian process should account for individual differences in both processes.