Vigilance or sustained attention refers to the ability of observers to maintain their focus of attention and remain alert to stimuli over prolonged periods of time (Warm, 1977, 1984: 1993; Davies and Parasuraman, 1982; Ballard, 1996). This aspect of human performance is an important concern for human factors/ergonomic specialists due to the critical role that vigilance plays in many operational settings, especially those involving automated human-machine systems. Advancements in technology have transformed the role of workers from that of active controllers to system executives who monitor the functioning of machines that do the work for them and intervene only in the event of potential problems (Sheridan, 1970, 1980). Consequently, vigilance is a critical component of human performance in a diverse array of work environments including military surveillance, air-traffic control, transportation security, nuclear power plant regulation, industrial quality control, and long-distance driving (Warm, 1984, 1993; Hartley et al., 1989; Howell, 1993; Satchel, 1993; Hancock and Hart, 2002). Vigilance also contributes to performance efficiency in medical settings, including x-ray and cytological screening and the inspection of anesthesia gauges during surgery (Weinger and Englund, 1990; Gill, 1996; Warm and Dember, 1998).