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Prevention of Cancer – 7 – Night Shift

                                                 Prevention of Cancer PART – 7
(All the articles published in past are available at
Question: In last part, we discussed role of Hormones to some extent in cancer causation. Breast uterus ovary prostate are the main cancers primarily driven by hormone changes. Role of hormones in obesity. Role of gut and microbiome in hormone control. Risk factors for breast cancer – most hormone related. New knowledge of risk factors for breast cancer: alcohol, even if only on weekend. Lack of exercise in young age, school college days.
We noted that many of our lifestyle changes have a big impact on our hormones. Such as having children late, not having children. And additional small risk from long term use of oral contraceptives. Obesity also has a dual impact. Hormone changes increase obesity and obesity increases hormone changes.
Ans: Breast cancer is Number 1 cancer in world today. Even in India, incidence is rising. Likely due to some of the factors mentioned above. For example, age at first delivery before 20 years provides the highest protection from breast cancer. But very few women now have children by this age. Additionally, exercise (physical work) in school and college days is now much lower. Time spent in outdoor activities is steadily reducing. With climate changes, it is becoming even more difficult.
Que: Thank you for covering the most important risk factors for hormone related cancers. There are many breast cancer related studies in literature. Any other risk factors that we should be aware of?
Ans: Yes. Modern science, especially these large epidemiological studies involving thousands of people and decades of follow up, have uncovered very important and some very intriguing information. For example, “night shift” increases risk of breast cancer. yes, night shift. In 2007, the International Agency for Research on Cancer (IARC) classified night shift work as probably carcinogenic for humans (group 2A), based on sufficient evidence from animal studies and limited evidence from epidemiological studies. They updated this in July 2019 with more confidence in the data showing night shift increases risk of certain cancers, mainly hormone related. Applicable to both men and women. Same opinion is of CDC, USA published in 2021.
Que: Are you serious? This is known since 2007 at least, and confirmed again and again. And published by most reputed organizations in the field of cancer world over. But how does night shift increase risk of any cancer?
Ans: Yes, this is not just one study. Multiple studies and repeatedly confirmed. Night shift is a risk factor for several diseases, not just cancer. “In these studies, night shifts seem to increase risk for type 2 diabetes, heart disease, stroke, metabolic disorders, and sleep disorders. Night shift workers might also have an increased risk for reproductive issues, such as irregular menstrual cycles, miscarriage, and preterm birth. Digestive problems and some psychological issues, such as stress and depression”. This is quoted directly from CDC, USA website. CDC, USA defines this risk factor mainly as below:
• Night shift work of at least 3 hours between midnight and 5:00 AM
• Frequent night work (3 or more nights per week)
• Long-term night shift work (10 or more years)
• Beginning night shift work in early adulthood (such as before the age of 30)
Before we go on, it is important to note that many youngsters today frequently sleep very very late, practically like a night shift. Note above that night shift beginning before age 30 is high risk. We have noted in first article of this series also: Risk of breast cancer is primarily determined by what the woman did in her life prior to age 30!
How does night shift increase health risks, including cancer. there are many theories. But below is one of the quote from CDC, USA.
“Circadian disruption can harm biologic systems that help prevent cancer. For example, in addition to promoting sleep, melatonin can also stop tumor growth and protect against the spread of cancer cells. Laboratory animals whose days and nights were disrupted in scientific experiments had reduced levels of melatonin and increased rates of cancer or tumor growth.”
Que: So what you are saying is that some of the most common diseases that we face today i.e. Cancer, Heart disease, Stroke, Diabetes, Infertility, Depression… have a link with night shift. And that being awake for at least 3 hours between midnight and 5 am is like night shift! This later type of “night shift” is nowadays extremely common, with people watching television or mobile or even working at home on laptop till late. Why is this not being highlighted enough??? This is a very important modifiable risk factor.
Ans: You are right. What we are doing is first step in this direction. Please spread this message. And start with your own family, friends, colleagues, neighbours – whoever is in your group – social media or otherwise.
July 16, 2022

Dr Chirag A. Shah; M.D. Oncology/Hematology (USA), 9998084001. Diplomate American Board of Oncology and Hematology. Ahmedabad.


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