To inform development of decisional support systems for the sleep deprived, this study examined the effect of sleep debt, time pressure and risk on the ability to use a decision aid. A total of 19 participants were tested when well rested and sleep deprived. Participants played computerised forms of Blackjack, which varied a 1- or 4-second response deadline, at two levels of risk, and could be supplied with online advice. Mean bets served as indications of confidence. Although confidence was less when play was fast or higher risk participants did not bet significantly less when sleep deprived, suggesting an impaired calibration of judgement that was supported by evidence of rallying. This failure to adjust confidence was accompanied by slower responses at low risk when sleep deprived. Sleep-deprived participants were less able to use decisional support under time pressure and made more errors without advice and time pressure.Practitioner summary: Decisional support is becoming more pervasive. To inform development of decisional support systems to assist the sleep deprived, an experiment considered the use of decisional support as a function of time pressure and risk. Advisory systems require processing and will be less efficacious under time pressure when sleep deprived.