People may suffer disruption to their biological rhythms for a number of different reasons. For example people may suffer sleep disruption due to things such as shift work and jet lag. Many industries require shift work to keep going 24 hours a day. Examples of such industries include airline crew, doctors and paramedics. People who work the night shift are required to be alert and night and so much therefor sleep through the day. This is a reverse of their natural body rhythms and goes against most available zeitgeber cues. Boivin et al (1996) found that night workers often experience a circadian trough of decreased alertness during their shifts� This is between the hours of midnight, when cortisol levels are at their lowest, and 4am, when core body temperature is at its lowest. This leads to the assumption that this is the period of time, during a night shift, in which most accidents may occur. A real world example that supports this assumption is the Chernobyl disaster. The Chernobyl disaster was a nuclear accident, purely down to human error, that occurred on 26 April 1986 at 3am, and therefor was during the circadian trough. Studies of sleep, such as the one by Boivin, may result in industries re-evaluating the shifts and shift patterns that their employees work.