Circadian rhythms are 24-hour cycles generated by internal molecular clocks found in nearly all organisms ranging from prokaryotes to primates. The evolutionary rationale behind circadian clocks is that organisms benefit from being able to predict changes in light and temperature fluctuations prior to their occurrence. For example, a nocturnal mammal may find a safe hiding place slightly before sunrise in order to avoid being exposed to predators. These behavioral changes are possible due to outputs from the cellular clocks to the tissue level, which result in endocrine signals that produce behavioral results. Historically, all animals (including humans) were exposed to constant light/dark cycles that only underwent gradual changes with the seasons. Current technology, however, has drastically changed the regularity and ratio of our light/dark cycle. A substantial amount of research suggests that these changes are not without consequence.
Individuals who work night shifts have been shown to have decreased life expectancies. This is in part due to increased risk of certain cancers, as well as increased of work-related deaths due to decreased cognitive ability in industrial settings. This phenomenon has also been replicated in controlled animal studies, where mice that were forced to shift their circadian phase showed decreased cognitive ability, and had a tendency to gain weight. Taken together, these data suggest that being exposed to artificial light and altered sleep schedules can have lasting negative effects on human health.
http://www.sciencedaily.com/releases/2011/02/110226214132.htm
http://www.guardian.co.uk/lifeandstyle/2009/mar/17/night-shifts-health
http://www.ias.ac.in/currsci/apr102007/890.pdf
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