The Hidden Cost of Night Shift: Why Circadian Misalignment Raises Breast Cancer Risk

Night shift work has always been part of many essential jobs. Hospitals don’t close. Planes don’t stop flying. Emergency services run around the clock. And for decades, women have been the backbone of these 24-hour systems.

But there’s a problem we don’t talk about enough.
Working against your natural body clock comes with a cost, and for women, that cost includes a higher risk of breast cancer.

We often hear about genes like BRCA(genes that produce proteins to repair damaged DNA), family history, diet, smoking, alcohol, and obesity. These matter. But one major factor is often ignored: circadian misalignment. In simple terms, your internal clock gets thrown off when you work through the night. And research is now showing just how serious that disruption is.

What the research shows

Back in 2007, the International Agency for Research on Cancer (IARC) classified “shift work involving circadian disruption” as a probable cause of breast cancer. At the time, some people questioned the decision. The evidence was there, but not strong enough for everyone.

Now it’s a different story.
A massive review has pulled together data from 33 studies and over 4.3 million women. And the results are clear: women who work night shift have a higher risk of breast cancer than women who never or rarely work nights.

The overall increase in risk sits at a hazard ratio of 1.20. That may look like a small number, but it’s a significant jump when you apply it to millions of women who work nights across healthcare, aviation, emergency services, and other essential jobs.

This isn’t fear-mongering. It’s what the data shows.
And women deserve to know the truth so they can protect their health.

Why night shift increases the risk

To understand why this happens, we need to look at what night work does inside the body.

Your circadian rhythm is your internal clock. It tells every cell in your body what time of day it is. Light is the main signal that sets this clock. When light hits the eyes at night, the brain gets confused. It thinks it’s daytime.

This stops or reduces the release of melatonin, which is normally produced only when it’s dark. And melatonin is far more important than most people realise. It isn’t just a “sleep hormone.” It plays a major role in cancer protection.

Here’s how:

• It helps repair DNA damage.

• It reduces oxidative stress.

• It acts as an anti-estrogen, which matters because many breast cancers grow in response to estrogen.

• It slows tumour cell growth and interferes with cancer cell signalling.

When night shift suppresses melatonin, your natural defences weaken. Estrogen becomes more dominant. Cells become more vulnerable. And over many years, that creates the conditions where breast cancer is more likely to develop.

There’s more. Long-term night shift work is linked to shorter telomeres (the protective caps on your DNA) and a weaker immune system due to disrupted sleep. Both of these add to the cancer risk.

Some women are at higher risk

The research doesn’t just look at “night shift workers” as a single group. It breaks the risk down further, and this is where things get even more important for women working long careers in shift work.

1. More nights per week = higher risk

Working 1–5 night shifts a week didn’t show a big jump in risk.
But working more than 5 nights per week did.
At that level, the hazard ratio jumps to 1.50.

In plain terms, the more often you work nights, the more disrupted your internal clock becomes, and the more your risk climbs.

2. The number of years you work nights matters

Risk increases with time spent doing night shift:

• 1–10 years → HR 1.09

• 11–29 years → HR 1.12

• 30+ years → HR 1.18

These numbers show what many shift workers already feel: long-term night shift takes a toll that slowly adds up.

3. Starting night shift before menopause increases risk

Women who began night shift before menopause showed a meaningful increase in risk (HR 1.17). Women who started after menopause did not show the same increase.

This makes sense.
Before menopause, estrogen levels are higher and far more active. When melatonin drops, estrogen’s effects become stronger. This creates the perfect environment for hormone-sensitive breast cancers to develop.

The types of cancer most affected

Night shift is most strongly linked to cancers driven by hormones:

• ER+ breast cancer

• PR+ breast cancer

• HER2+ breast cancer

These cancers respond to estrogen and progesterone. Because melatonin acts as a natural anti-estrogen, any drop in melatonin removes part of that protective effect. This explains why receptor-positive cancers show the strongest link.

Night shift did not show a clear link with receptor-negative cancers. Again, this supports the hormonal pathway as the main mechanism.

What needs to change

This isn’t about blaming shift workers. It’s not about telling women to quit their jobs. Night shift is part of society, and many women rely on these roles.

But we can change how we work and how we protect people who work against their body clock.

This research should push employers, health services, and governments to:

• Create better-designed rosters

• Reduce long runs of night shift

• Avoid rotating backwards

• Allow recovery time between shifts

• Offer education on circadian health

• Recognise night shift as a genuine occupational health risk

And for individual shift workers, this information is power.
When you understand the risk, you can take steps to reduce it, through better sleep timing, controlled light exposure, strategic use of morning light, managing consecutive nights, and keeping regular routines on days off.

Circadian health is not a “nice to have.” It’s a foundation of long-term wellbeing.

Night shift doesn’t just make you tired.
It alters your biology at the cellular level.
And for women, especially younger women, that disruption carries a real and measurable breast cancer risk.

The more we talk about this, the more we can push for safer shift systems and better support for the people who keep our 24-hour world running.

I have made it my mission to support shift-working women.  

If you need help in your shift working life to combat the negative effects of shift work I will help you.    

There are simple habits and routines that will support you in your shift working life.

Reach out today and let me support your future health.  Your future health is counting on it.

Reference: 

Hong J, He Y, Fu R, et al. The relationship between night shift work and breast cancer incidence: A systematic review and meta-analysis of observational studies. Open Med (Wars). 2022;17(1):712-731. Published 2022 Apr 8. doi:10.1515/med-2022-0470

About Roger Sutherland

As a coach and advocate for shift workers, my goal is to provide practical, evidence-based strategies that empower individuals to thrive in their roles. By understanding and addressing the challenges of shift work sleep disorder, shift workers can achieve better health outcomes and lead more fulfilling lives both on and off the job.

Note:

• I also run Nutrition, Health & Wellbeing Seminars for shift working environments.