Humans have evolved in a rhythmic environment and display daily
(circadian) rhythms in physiology, metabolism and behaviour that are
in synchrony with the solar day. Modern lifestyles have compromised
the exposure to bright light during the day and dark nights, resulting
in the desynchronisation of endogenously generated circadian
rhythms from the external environment and loss of coordination
between rhythms within the body. This has detrimental effects on
physical and mental health, due to the misregulation and uncoupling
of important cellular and physiological processes. Long-term shift
workers who are exposed to bright light at night experience the
greatest disruption of their circadian rhythms. Studies have shown
an association between exposure to light at night, circadian rhythm
disruption and an increased risk of cancer. Previous reviews have
explored the relevance of light and melatonin in cancer, but here we
explore the correlation of circadian rhythm disruption and cancer in
terms of molecular mechanisms affecting circadian gene expression
and melatonin secretion.