From 'How Airliners Fly' by Julien Evans:
Human factors
Humans differ from machines in that they do not always perform their activities in a predictable manner. To a very large extent this shortcoming can be overcome by training. In the operation of complex machinery such as airliners predictability is enhanced by the use of standard operating procedures (SOPs). Pilots and cabin crew come in all shapes and sizes and with differing personalities. By following SOPs, efficient and safe operation of the aircraft can be guaranteed regardless of the personalities of the individual crew members.
Another human failing is physiological and psychological variability. Persons unsuited by nature to the roles of pilot or cabin crew are normally filtered out by selection procedures. But even those who are chosen will not perform perfectly every time they go to work. However, professionalism demands that they do their best even if they are tired or angry or depressed, and doing their best means meeting or exceeding the minimum standards set.
Passengers too have a contribution to make. For example, they can take the trouble to pay attention to the safety demonstration before take-off and to read the safety card - just in case. And by reciprocating the courtesy and consideration which the cabin crew extend to them they can help build a comfortable ambience in the cabin, to the benefit of all parties. Remember that the steward or stewardess who is serving you your meal may also be the one who helps to save your life in an emergency.
The trickiest human problem is that of crew fatigue, which can be defined as debilitating tiredness. A pilot who is tired but not overtired will be able to perform his or her tasks properly, taking extra care to compensate for lack of freshness. But the very ability to self-monitor is degraded by fatigue and herein lies the danger. In other words a fatigued pilot might not be aware that he or she is performing badly. There is a degree of protection in the presence of the other pilot in the crew, one of whose primary duties is to draw attention to errors or omissions made by their colleague. If both pilots in a crew are fatigued the implications for flight safety are obvious, particularly if their workload is compounded by factors such as technical problems or bad weather or making an approach into a difficult airport.
For this reason the regulatory authorities set maximum permitted durations of duty period for pilots. These maxima are governed by factors such as the time of day (or night) the duty commences and how many legs (take-offs and landings) are to be carried out. The cabin crew must have similar protections because they too might have to carry out emergency duties at the end of the duty period. For very long flights, the rules demand the presence of an extra pilot in the crew to allow in-flight relief for the other two. Minimum lengths of rest periods between duties are also specified.