Performance on many decision-making tasks is underpinned by metacognitive monitoring, cognitive abilities, and executive functioning. Fatigue-inducing conditions, such as sleep loss, compromise these factors, leading to decline in decision performance. Using a 40-hr sleep deprivation protocol, we examined these factors and the resulting decision performance. Thirteen Australian Army male volunteers (aged 20-30 years) were tested at multiple time points on psychomotor vigilance, inhibitory control, task switching, working memory, short-term memory, fluid intelligence, and decision accuracy and confidence in a medical diagnosis-making test. Assessment took place in the morning and night over two consecutive days, during which participants were kept awake. Consistent with previous work, cognitive performance declined after a night without sleep. Extending previous findings, self-regulation and self-monitoring suffered significantly greater declines immediately after the sleepless night. These results indicate that the known decline in complex decision-making performance under fatigue-inducing conditions might be facilitated by metacognitive rather than cognitive mechanisms.