ological and behavioral functions that repeat at regular intervals ranging from minutes to months. Circadian rhythms, also known as biological rhythms approximating the 24-hour earth's rotation, are generated by an internal clock or pacemaker in the brain which is synchronized to daily time cues in the environment such as the light-dark cycle (Liskowsky, 1992).
Functions that vary with a circadian rhythm include body temperature, sleep and wakefulness, the release of many hormones, alertness, and memory (Liskowsky, 1992). Any work schedule that requires people to work when they would normally be sleeping or to sleep when they would normally be awake can disrupt their circadian rhythms. What results in many instances is an increased potential for workplace accidents, mistakes, injuries, and enhanced risk of illness on the part of workers (Henderson & Burt, 1998). In addition, disruption of circadian rhythms can result in a decline in the ability to perform tasks, the persistent symptoms of fatigue, difficulty in concentrating, and sleep-deprivation related headaches and feelings of physical imbalance (Waterhouse, 1993).
The field of human factors developed in the United States in 1957 with the founding of the Human Factors Society (Hawkins, 1993). The field is firmly rooted in aviation, in which shift work is endemic and in which frequent changes of time zones are commonplace for pilots and flight crew members. Most of the human factors research directed toward the aviation industry has tended to be focused on cockpit and flight crew issues. Hawkins (1993) and the International Civil Aviation Organization (1992, 1993, 1995) have commented that it has become increasingly apparent that the public's safety in flight rests on the proper conduct of three sets of activities: design, operation, and maintenance. Maintenance tasks, many of which are performed by shift workers who could be negatively be affected by fatigue distur...