It is perfectly normal for people to become tired. Nearly everyone experiences fatigue, however, some professions, such as aviation, medicine or the military, demand precise, rapid and well-informed decision making as well as communication with little margin for error.
Concern about the potential for fatigue to negatively affect human performance in the aviation context extends back decades, with both airlines and pilots agreeing that fatigue is an issue posing a very troubling threat to millions of passengers as well as valuable cargo supplies that are carried by various means of air transport each day.
Obviously, this is an acceptable risk which cannot be fully excluded; however, this problem is not given enough credits. Exhaustion in the cockpit can be deadly as seen in 2009 when an American commuter plane crashed killing everyone on board. That accident was blamed in part on fatigue.