Associations of Circadian Disruption of Sleep and Nutritional Factors with Risk of Cancer

Abstract

Background: Daily entrainment of the human circadian clock is important for good human health. In previous studies, shift work has been linked to higher risk of chronic diseases, including certain types of cancers. Exposure to light at night suppresses the physiologic production of melatonin, a hormone that has antiproliferative effects on intestinal cancers. In the present review, we examine the available evidence on sleep disruption, changes in nutrient intake and nutritional factors and risk of cancers. Methods: Internet search of PubMed and discussion with colleagues. Results: Recent studies indicate that night shift work appears to have independent influence on the function of the endocrine system, gastrointestinal tract and circadian brain function. Sleep disruption enhances cortisol secretion and ghrelin release from the stomach and decreases melatonin and leptin which interfere with functioning of beta cells of pancreas. Apart from biological dysfunctions, behavioral changes, increased intake of refined carbohydrates, w-6 fats and low w-3 fats, physical inactivity, excess of tobacco and alcoholism appear to be common among night shift workers. Leptin signals the brain to feel satiety whereas ghrelin, produced in the stomach, signals hunger. Recent studies also indicate that sleep-deprived individuals with hormonal changes have greater cravings for sweet and fatty foods. Apart from this, stress hormone cortisol, which increases with sleep deprivation also contribute to hunger. In addition to altered hormone levels, late night awakening provides greater opportunity to eat, smoke and drink alcohol and eating often includes high-caloric foods. Epidemiological studies indicate that sleep disruption may be associated with obesity and other chronic diseases including cancers. Since electric light at night has adverse effects among night shift workers compared to day shift workers, it has been proposed that a portion of the high and rising risk of breast and prostate cancer worldwide may be because of night shift work. The suppression of melatonin by exposure to light at night may be one reason for the higher rates of breast, prostate and colorectal cancers in the developed world. Suppression of nocturnal melatonin by exposure to light at night results in lack of protection by melatonin on cancer cell receptor sites which allows the uptake of linoleic acid (LA) which in turn enhances the growth of cancer cells. Melatonin is a protective, oncostatic hormone and strong antioxidant having evolved in all plants and animals over the millennia. It is possible that rotating night shift at least three nights per month for 15 or more years may increase the risk of colorectal cancer and other cancers. Conclusions: Experimental evidence and limited human evidence allowed the International Agency for Research on Cancer (IARC) to classify circadian disruption of sleep, as a probable human carcinogen, group 2A. Behavioral changes, intake of fast foods, physical inactivity, excess of tobacco and alcoholism are common among night shift workers which may also apart from deficiency of melatonin.