Shift workers who work at night, late evenings or early mornings will experience some degree of sleep deprivation leading to a fall in alertness during the duty period. For night workers, this is exacerbated by naturally falling performance that is driven by the circadian rhythm. With the exception of night workers on oil rigs, there is unlikely to be any improvement due to adaptation. The degree of sleep deprivation varies with the individual and becomes more pronounced with aging. Although counter measures such as napping before and during duty can help, it is likely that some adverse effects will persist. The consequence of a fall in alertness leads to slowed reaction times and reduced vigilance, poor decision making and lack of communication. In extreme cases this may contribute to incidents and accidents as evidenced by Exxon Valdiz and the Challenger space mission disaster.