It has been estimated that 4-7% of civil aviation incidents and accidents can be attributed to fatigued pilots. "In the last 16 years, fatigue has been associated with 250 fatalities in air carrier accidents." Robert Sumwalt, NTSB vice chairman, said at an FAA symposium in July. Symptoms associated with fatigue include slower reaction times, difficulty concentrating on tasks resulting in procedural mistakes, lapses in attention, inability to anticipate events, higher toleration for risk, forgetfulness, and reduced decision-making ability. The magnitude of these effects are correlated to the circadian rhythm and length of time awake. Performance is affected the most, when there is a combination of extended wakefulness and circadian influences. A Federal Aviation Administration (FAA) study of 55 human-factor aviation accidents from 1978 to 1999, concluded accidents increased proportionally to the amount of time the captain had been on duty. The accident proportion relative to exposure proportion rose from 0.79 (1–3 hours on duty) to 5.62 ( more than 13 hours on duty). This means that "5.62% of human factors accidents occurred to pilots who had been on duty for 13 or more hours, where only 1% of pilot duty hours occur during that time."  In another study by Wilson, Caldwell and Russell, participants were given three different tasks that simulated the pilot's environment. The tasks included reacting to warning lights, managing simulated cockpit scenarios, and conducting a simulated UAV mission. The subjects' performance was tested in a well-rested state and again after being sleep deprived. In the tasks that were not as complex, such as reacting to warning lights and responding to automated alerts, it was found that there was a significant decrease in performance during the sleep deprived stage. The reaction times to warning lights increased from 1.5 to 2.5 seconds, and the number of errors doubled in the cockpit. However, tasks that were engaging and required more concentration were found to not be significantly affected by sleep deprivation. The study concluded that "...fatigue effects can produce impaired performance. The degree of performance impairment seems to be a function of the numbers of hours awake and the 'engagement' value of the task."  One United States Air Forces study found significant discrepancies regarding how fatigue affects different individuals. It tracked the performance of ten F-117 pilots on a high-fidelity flight simulator. The subjects were sleep deprived for 38 hours and their performance was monitored over the final 24 hours. After baseline correction, the systematic individual differences varied by 50% and concluded that fatigue's effect on performance varied drastically among individuals. The first step to understanding the critical impact fatigue can have on flight safety is to quantify it within the airline environment. An airline's management often struggles to balance rest with duty periods because it strives for maximum crew productivity. However, fatigue comes as a limitation needing increasing consideration. A study by Reis et al. investigated the prevalence of fatigue on a group of Portuguese airline pilots. 1500 active airline pilots who had all flown within the past six months received a questionnaire. Out of the population, 456 reliable responses were received. A pretest was conducted to determine the viability of the fatigue scale adopted during the test, called Fatigue Severity Scale (FSS). The purpose of the validation survey was to set a benchmark (i.e. FSS=4) on an acceptable level of fatigue for the Portuguese culture. The scale ranged from 1 meaning no fatigue to 7 being high. Participants had one month and a half to respond to the inquiry. Results on physical fatigue found that 93% of short/medium haul pilots scored higher than 4 on the FSS while 84% of long-haul pilots scored greater than 4. Mental fatigue found short/medium haul at 96% and long haul at 92%. The Questionnaire als