In most studies, an excess risk of breast cancer was found mainly among women who had worked night shifts for many years or at a high frequency, or who had worked a large number of night shifts over their lifetimes. Moreover, studies among nurses and other night shift workers showed increased risk of breast cancer that was unexplained by reproductive history, lifestyle factors, body mass index, or socioeconomic status. Laboratory animals whose days and nights were disrupted in scientific experiments had reduced levels of melatonin and increased rates of cancer or tumor growth.Īnimal studies have shown that exposure to light at night led to the growth of breast or mammary-gland cancer. For example, in addition to promoting sleep, melatonin can also stop tumor growth and protect against the spread of cancer cells. The connection between circadian disruption, melatonin, and cancerĬircadian disruption can harm biologic systems that help prevent cancer. When our sleep patterns or lightness and darkness cues are severely disrupted-as can happen with night shift work, travel across multiple time zones, or exposure to light during our normal sleeping hours-we can develop what is referred to as “circadian disruption.” Circadian disruption means our circadian rhythms are not working together, which can make us feel ill, increase our risk for poor health, and increase our chances for making mistakes that risk our safety and the safety of others. Our master clock in the brain controls the timing of our circadian rhythms so they work together: this harmony is important for a healthy functioning body. When darkness falls, the hormone melatonin is secreted by the pineal gland in the brain and is produced all night long, promoting sleep. Light stimulates areas of the brain to tell our bodies to be awake, active, and hungry. The timing of our daily rhythms is strongly influenced each day when light is first detected by our eyes. Many of our biologic systems – including our sleep and wake cycle, our appetites and digestive patterns, our body temperatures, and even our moods – follow daily patterns, or “circadian rhythms.” These cycles are set by circadian clock genes that are found in nearly every cell in our bodies. Many workers might wonder how night shift work could be related to cancer, and what workers can do to stay healthy. In the recent NTP report, it was concluded that there is “high confidence” that persistent night shift work that results in circadian disruption can cause human cancer, and IARC concluded that night shift work is “probably carcinogenic to humans.” These conclusions are based on evidence from human studies of breast and prostate cancer, studies of laboratory animals, and research into the mechanisms of how cancer develops. Both agencies reviewed existing studies of night shift work and cancer and both convened experts in the fields of human, animal, and basic science research. This report follows a similar evaluation released in July, 2019 by the International Agency for Research on Cancer (IARC) (2), an update of their 2007 report (3). The National Toxicology Program (NTP) recently released a report about how persistent night shift work is related to cancer risk (1).
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