Night Shift Work and Cancer Risk: What Research Says

Millions of people in India work while others sleep, from nurses and doctors to call centre staff, police officers, drivers, pilots and factory workers. A common worry is whether night shift work can raise the risk of cancer. The short answer is that the World Health Organization’s cancer agency considers it a probable cause of cancer, but the evidence in people is still limited. This guide explains what the research says, why the body clock matters and what shift workers can do to protect their health.

Our old post on this topic made strong claims, such as exact percentage increases in risk, without clear sources. We have rewritten it using the findings of the International Agency for Research on Cancer, known as IARC.

What IARC Says About Night Shift Work

IARC is part of the World Health Organization. It reviews the evidence on possible causes of cancer and places them into groups. In 2007, it classed shift work that disrupts the body clock as probably carcinogenic to humans, which is Group 2A.

In 2019, a new IARC working group looked again at the evidence and kept night shift work in Group 2A. The full report, Volume 124 of the IARC Monographs, was published online in June 2020. IARC defines night shift work as work during the usual sleeping hours of the general population, and it includes long flights across time zones.

The working group based its decision on three types of evidence:

• Limited evidence in humans for cancers of the breast, prostate, colon and rectum.
• Sufficient evidence in experimental animals exposed to disrupted light and dark cycles.
• Strong evidence from experimental animals about how such disruption could lead to cancer.

What Limited Evidence Means

Limited evidence means some studies of workers found a link between night shifts and these cancers, but other explanations could not be ruled out. For example, people who work nights may differ from day workers in other ways, such as diet, smoking, exercise or access to health checks. Some studies found a link, and others did not.

What Group 2A Means

It is easy to misread these groups. IARC groups describe how strong the evidence is that something can cause cancer. They do not say how large the risk is. Group 2A also includes eating red meat and drinking very hot drinks above 65 degrees Celsius. So the label tells us night shift work is a likely hazard, not that every night worker will get cancer, or that the risk is as high as from smoking.

Who Works Night Shifts

Night work is essential in many fields. IARC lists health care, manufacturing, transport, retail and services as sectors where work must continue 24 hours a day, seven days a week. In India, this includes hospital staff, security guards, police, railway and bus crews, airline staff, delivery workers, journalists and the large number of people in call centres and technology firms who work with clients in other time zones.

Why the Body Clock Matters

Your body runs on an internal clock called the circadian rhythm. It follows a cycle of about 24 hours and tells you when to feel sleepy, when to feel alert, and when to release many hormones. Light is its main signal. Bright light in the morning tells the body that the day has begun, and darkness in the evening tells it to prepare for sleep.

IARC notes that disruption of these natural rhythms is the most marked effect of night shift work. Working at night means being awake, eating and facing bright light at times when the body expects to rest. Then the worker tries to sleep during the day, when the body clock is pushing for wakefulness and the world outside is bright and noisy.

The Role of Melatonin

One hormone that researchers often study is melatonin. The brain releases it in the evening and at night, and it helps signal that it is time to sleep. Light at night suppresses melatonin, and the stronger and longer the light, the greater the effect. Scientists are studying whether repeated melatonin suppression and body clock disruption could affect how cells grow and repair themselves, but this is still being researched in people.

Other Health Effects Linked to Shift Work

Cancer is only one of the concerns. Research on shift workers has linked their schedules to several other problems:

• Poor sleep: Daytime sleep tends to be shorter and lighter than night sleep.
• Shift work sleep disorder: Ongoing trouble sleeping, or heavy sleepiness at work. It is estimated to affect between 10 and 40 percent of shift workers.
• Weight and blood sugar: Shift work has been associated with obesity, insulin resistance and type 2 diabetes.
• Mood: Higher rates of depression and anxiety have been reported.
• Accidents: Tired workers are more likely to make mistakes, including on the drive home.

Many of these problems are linked, since poor sleep can affect appetite, mood and alertness. Adults are generally advised to get seven or more hours of sleep a day, which can be hard for night workers.

How Shift Workers Can Protect Their Health

You may not be able to change your job, but there are practical steps that can help. These are general tips, not medical advice.

Better Daytime Sleep

• Try to sleep at the same time after each shift, even on some days off.
• Make the bedroom dark with blackout curtains or an eye mask.
• Keep it quiet and cool. Earplugs or a fan for steady background noise can help.
• Tell family members your sleep hours and silence your phone.
• Avoid tea, coffee and energy drinks in the last few hours before you plan to sleep.
• Do not use alcohol to help you sleep, as it disturbs sleep quality.

For more ideas, see our posts on tips for proper sleep and habits that affect your sleeping pattern.

Light and Timing

Bright light during your shift can help you stay alert, while reducing bright light on the way home can make it easier to fall asleep afterwards. Some workers wear sunglasses when travelling home in the morning. A short nap before a night shift can also reduce sleepiness at work.

Food and Activity

Plan regular meals rather than snacking all night. Lighter meals during the night shift and a balanced meal before sleep may feel better than heavy, oily food in the early hours. Regular physical activity and staying at a healthy weight help health in general, and these are among the proven ways to lower cancer risk.

Health Checks and Screening

Because the possible link with cancer is still being studied, the steps with the strongest evidence are the same for everyone: do not smoke, limit alcohol, stay active, eat plenty of vegetables, fruit and whole grains, and keep a healthy weight. Read more in our post on ways to reduce your cancer risk.

Go for regular health checks and ask your doctor which cancer screening tests are right for your age and family history. Women can learn the signs of breast cancer in our guide to breast cancer causes, symptoms and treatment, and should see a doctor about any new lump or change.

Safety on the Road

Drowsy driving is dangerous. If you feel very sleepy after a shift, take a short rest before driving, share a ride, or use public transport if you can.

What Employers Can Do

Workplaces also play a part. Occupational health guides often suggest limiting the number of night shifts in a row, giving enough rest between shifts, rotating schedules forward from mornings to evenings to nights, providing good lighting and healthy food at night, and offering health checks. Workers with sleep problems should be able to raise them without fear.

When to See a Doctor

See a doctor if you have constant trouble sleeping, fall asleep at work or while driving, feel low for weeks, or notice warning signs such as an unexplained lump, weight loss, a change in bowel habits or blood in your stool or urine. Do not start sleeping pills or supplements without medical advice.

The IARC review covered many types of night workers, including nurses, factory staff and airline crew who regularly fly across time zones.

What We Changed From the Old Post

We rewrote the post around IARC’s findings. We removed the unsourced 30 percent breast cancer figure, the claim that long term night work causes lung cancer, and a list of other illnesses that had no source. We explained what Group 2A means, added the science of the body clock and melatonin, and added practical tips for workers and employers.

Frequently Asked Questions

Does night shift work cause cancer?

IARC classes it as probably carcinogenic to humans. The evidence in people is limited, so it has not been proven to cause cancer.

Which cancers have been linked to night shifts?

IARC found limited evidence for cancers of the breast, prostate, colon and rectum.

Can I lower my risk if I work nights?

Yes. Protect your sleep, do not smoke, limit alcohol, stay active and go for regular check ups.

Is the risk the same for everyone?

No one knows yet. Researchers are still studying how the number of years, the type of schedule and personal factors affect any risk.