Pilot fatigue is described by the International Civil Aviation Organization as "a physiological state of reduced mental or physical performance capability resulting from sleep loss or extended wakefulness, circadian phase, or workload." As scary as it is to think about, this is definitely an important issue that increases the chance of pilot error, also known as cockpit error, which includes mistakes, oversights, lapses in judgment, gaps in training, adverse habits, and failures to exercise due diligence in a pilot's duties. Airlines avoid this at all costs by limiting the number of hours pilots are allowed to fly over varying periods of time.
The image above shows an experimental device that was designed to counteract pilot fatigue. The prone position bed helped them to combat gravitational forces experienced during flight. This was one of the more extreme measures taken, while the Federal Aviation Administration has studied the illness for years. 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. It's estimated that 4-7% of civil aviation incidents and accidents can be attributed to fatigued pilots.