The present study aimed to systematically investigate the effects of elevated fatigue levels on the ability to self-monitor performance. Eighteen participants, aged 19-26 y, remained awake for a period of 28 h. Neurobehavioural performance was measured at hourly intervals using four tests from a standardized computer test battery. From these four tests, six measures of performance were obtained: grammatical reasoning (accuracy and response latency); vigilance (accuracy and response latency); simple sensory comparison and tracking. In addition, before and after each test, participants completed visual analogue scales which required them to rate their alertness level and the speed and accuracy of their performance. Individual test results for both self-ratings and neurobehavioural performance were converted to z-scores. Planned comparison analysis indicated that scores on four of the six performance measures decreased significantly as hours of wakefulness increased. Similarly, predicted performance scores for all six measures of performance decreased significantly. Analysis revealed moderate correlations between predicted and actual performance for the four parameters affected by fatigue. Furthermore, moderate to high correlations were found between all six performance parameters and their respective post-test self-ratings. In addition, moderate to high correlations were found between predicted performance and alertness. Taken together, these findings suggest that as fatigue levels increase, subjects globally assess performance decrements. Results suggest that subjective alertness may in part mediate an individual's global assessment of performance.