The National Toxicology Program (NTP) recently released a report about how persistent night shift work is related to cancer risk. This report follows a similar evaluation released in July, 2019 by the International Agency for Research on Cancer (IARC), an update of their 2007 report. 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. 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. Many workers might wonder how night shift work could be related to cancer, and what workers can do to stay healthy.

Biologic clocks and circadian disruption

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. The timing of our daily rhythms is strongly influenced each day when light is first detected by our eyes. Light stimulates areas of the brain to tell our bodies to be awake, active, and hungry. When darkness falls, the hormone melatonin is secreted by the pineal gland in the brain and is produced all night long, promoting sleep. 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 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.

The connection between circadian disruption, melatonin, and cancer

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.

Animal studies have shown that exposure to light at night led to the growth of breast or mammary-gland cancer. 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. 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.

What does this mean for workers?

Although we don’t know specific cancer risks potentially associated with night shift work, the NTP and IARC reports both suggest that people who regularly work night shifts appear to be at highest risk. Specifically:

• 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)
• Other factors that are more common among night shift workers might contribute to the risk of cancer:
  • Workplace and social stressors
  • Lifestyle behaviors, such as smoking, higher than recommended alcohol consumption, poor diet, not enough physical activity, and being overweight or obese
  • Decreased exposure to sunlight and lower levels of vitamin D

What other health effects are associated with night shift work?

In addition to cancer, night shift work has been associated with 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 are more common among night shift workers. The fatigue associated with nightshift can lead to injuries, vehicle crashes, and industrial disasters.

What can night shift workers do to stay healthy?

• Take a NIOSH online training program on shift work to learn about individual and workplace strategies to better cope with working at night.
• Get regular health checkups and tell your doctor if you have any of the following possible symptoms of night shift work:
  • severe fatigue or sleepiness when you need to be awake
  • trouble with sleep, stomach or intestinal disturbances
  • irritability or bad mood
  • poor performance (frequent mistakes, injuries, vehicle crashes, near misses, etc)
  • unexplained weight gain or loss
• Shift workers should practice good health behaviors to combat the effects of night shift work:
  • get enough sleep
  • eat a nutritious diet
  • exercise regularly
  • avoid using tobacco
  • limit alcohol consumption
• Check the NIOSH website periodically for updates on night shift work and health.