Fatigue

Acute fatigue: How you physically and mentally feel at the end of a long day.
Chronic fatigue: Long-term fatigue which has serious physiological and psychological effects.
Performance degradation due to acute fatigue can be correlated to blood alcohol level.

• 17 hours awake = 0.05% blood alcohol level
  
• 20 hours awake = 0.10% blood alcohol level

We clearly would not fly if we were intoxicated. Why would we fly if our judgment was equally impaired simply because we are tired?  One of the best mitigation strategies is to take a short nap. The effects of acute fatigue can be temporarily mitigated with the strategic use of caffeine.
 
The longer a person is awake, the sleep urge and need becomes stronger. The pressure to sleep grows more intense as the day progresses, then dissipates when one sleeps at night.

As one extends wakefulness into routine sleep hours, the urge to sleep increases.  The primary factor that affects alertness is the number of hours continuously awake. At about the 16 hour mark, sleepiness begins to affect performance of the average individual. 

Studies show that after 18 hours of wakefulness, participants showed a 30% decrement in performance and problem-solving ability.

Chronic fatigue can be caused by multiple back-to-back long days with less than the required sleep each night. This leads to serious performance degradations. Performance degradations include the following:

• Reaction time decreases by 25%.
• Mood changes 100% (from good to bad.)
• Attention lapses increase five-fold.
• Lethargy and apathy increase by 50%.
• Judgement and decision making decrease by 50%.
• Memory decreases by 20%.
• Communication decreases by 30%.
  

Those performance degradations in the cockpit have manifested themselves in the following ways:

• Lower standards of performance unconsciously become acceptable.
• Attentional resources are difficult to divide effectively.
  
• Performance becomes inconsistent.
• The ability to reason logically is impaired.