You might expect shift work to cause insomnia, irritability, absenteeism, an increase in accidents on the job, and so on, and it does–especially for people on the third shift, which typically occurs from 10 p.m. to 6 a.m. However, when the graveyard shift disrupts circadian rhythms, it not only causes many expectable problems, but also increases the risk of cancer. Consequently the World Health Organization has tentatively classified the graveyard shift as a possible carcinogin.