Fatigue is caused by prolonged periods without sleep, long work hours, or working at times that disrupt circadian rhythms. It leads to slower brain activity, poorer performance, and increased risk of errors. Fatigue is difficult to directly measure but signs include sleepiness, lack of motivation, and mood changes. Countering fatigue requires optimizing sleep through consistent sleep schedules, exercise, and avoiding substances like caffeine and alcohol before bed. For shift workers, strategies include reducing mental demands, scheduling breaks to coincide with natural sleep periods, and brief naps if allowed.