4:00 a.m. and 1:23 a.m. What do they have in common?

Shortly after 4:00 a.m.on March 28, 1979a malfunction occured at the Three Mile Island nuclear power plant in Harrisburg, PA, which was followed by errors made by workers on the night shift.

At 1:23 a.m. on April 26, 1986, mistakes by night shift workes at the nurclear reactor in chernobyl, USSR,led to a catastrophic nuclear disaster.

A paper by Kenneth N. Fortson in the Monthly Labor Review found that accidents that result in injuries are much more likely to occur at night than in daytime hours and the best explanation is that working at night is dangerous due to disturbance of the body's biological clock or circadian rhythms.

Fortson used data from Texas workers' compensation claims which showed that the injury rate is higher during off-hours late at night than during the regular 9-5 shift. He first looked at three possible explanations: a younger and less experienced workforce late at night; more dangerous jobs being done at night; or fatigue because night workers work longer hours. The study found that none of these were good explanations of the higher late-night injury rate.

So what was the answer to the problem? Disturbance of the body's 24-hour biological patterns known as circadian rhythms. Studies have shown that the lowest point of the circadian cycle occurs in the early morning hours and can influence the short-term memory, reaction time and visual vigilance. The article concluded that there was

strong evidence that workes are not optimally alert during night shifts, contribuing to hazardous work conditions for themselves as well as their fellow employees.

He notes that there are two basic reasons that workers work at night. Some workers would rather work at night, for example single mothers, because of lack of affordable childcare during the day. Business often feel they can increase the productive capacity of plant with night shifts. Fortson warns, however, that there is a tradeoff: night work is inherently more hazardous than day work.