Fatigue is a general term which is difficult to define medically. Usually thought of simply as "tiredness," fatigue may be more aptly described as a depletion of body energy reserves, leading to below-par performance. Because fatigue lowers your efficiency as a pilot, you should understand its causes and prevention.

Fatigue falls into two broad categories: (1) acute fatigue (short-term), and (2) chronic fatigue (long-term).
 
Chronic fatigue, extending over a long period of time, usually has psychological roots. (An underlying disease is sometimes responsible, however). Continuous stain on your job, for example, can produce chronic fatigue. You may experience this condition in the form of weakness, tiredness, palpitations of the heart, breathlessness, headaches, or irritability. Sometimes chronic fatigue even creates stomach or intestinal problems and generalized aches and pains throughout the body. When the condition becomes serious enough, it can lead to emotional illness. If you suspect that you are suffering from chronic fatigue, consult your doctor. Self-help cures are rare. Above all, don't fly!

Acute fatigue, on the other hand, is short-lived and is a normal occurrence in everyday living. It is the kind of tiredness you feel after a period of strenuous effort, excitement, or lack of sleep. Rest after exertion and 8 hours of sound sleep ordinarily cures this condition.

A special type of acute fatigue, called "skill fatigue," is worth mention here because pilots are especially susceptible to it. Skill fatigue has two main effects upon your performance:

• Timing disruption - You appear to perform a task as usual, but the timing of each component is slightly off. This makes the pattern of the operation less smooth, because you perform each component as though it were separate, instead of part of an integrated activity.
   

• Disruption of the perceptual field - You concentrate your attention upon movements or objects in the centre of your vision and neglect those in the periphery. This may be accompanied by loss of accuracy and smoothness in control movements.

   Acute fatigue has may causes, but the following are among the most important for the pilot:

• Mild hypoxia (oxygen deficiency).
   

• Physical stresses produced by the aircraft, such as fighting severe turbulence, icing conditions, malfunctioning of the equipment.
   

• Psychological stress, some of it emotional and some resulting from the demanding intellectual activity required for successful flight operations.
   

• Depletion of physical energy resulting from psychological stress. Sustained psychological stress accelerates the glandular secretions and prepare the body for quick reactions during an emergency. These secretions make the circulatory and respiratory systems work harder, and the liver releases energy to provide the extra fuel needed for brain and muscle work. When this reserve energy supply is depleted, the body lapses into generalized and severe fatigue.

Acute fatigue can be prevented by a proper diet and by adequate rest and sleep. A well balanced diet prevents the body from having to consume its own tissues as an energy source. Adequate rest maintains the body's store of vital energy. You can sleep best in quiet, comfortable surroundings. Excitement and worry will diminish the benefits of sleep. As a pilot, you should get approximately 8 hours of sleep a night. If you are especially tired, tense, or ill, you will need more.

Keeping your body in top physical condition makes you less susceptible to fatigue. In addition to getting regular exercise, you should avoid overweight. Obesity lowers your flight performance, taxes your body, and shortens your life,