Patients in rooms with windows facing east received beneficial morning doses of light therapy without anyone intending, or even realizing, that at the time. Since then, controlled studies have confirmed this explanation. (See below for more information.)
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.