What you may have heard
Some studies suggest that people who work night shifts have a higher risk of cancer, especially breast cancer in women. The theory is that disrupting the body’s natural sleep–wake cycle (circadian rhythm) and suppressing melatonin may promote tumor development.
What science tells us
In 2019, the International Agency for Research on Cancer (IARC) classified “night shift work that involves circadian disruption” as Group 2A (probably carcinogenic to humans), based on limited evidence in humans, sufficient evidence in experimental animals, and strong mechanistic evidence tied to circadian disruption (IARC).
Epidemiological Evidence
- Meta-analyses of observational studies provide mixed results. A 2021 systematic review found that short-term night-shift work (<10 years) was associated with a 13% higher breast cancer risk (RR ~1.13), whereas long-term night-shift work (≥10 years) did not show a statistically significant increase in risk. (PubMed)
- A 2024 dose-response meta-analysis showed that breast cancer risk increased gradually with longer exposure. After one year of night shift work, the relative risk rose by about 0.4%, and by 13% after 30 years in cohort studies; in case-control studies, the increase was around 88% after 30 years. (PubMed)
Laboratory Evidence / Supporting Evidence
Animal studies demonstrate that disrupting the light–dark schedule accelerates tumor growth and alters clock gene expression. Circadian disruption impairs DNA repair and hormonal regulation, creating conditions that can favor cancer initiation (NLM).
IARC Carcinogen Classification
IARC designates night shift work involving circadian rhythm disruption as Group 2A – Probably carcinogenic to humans, acknowledging strong mechanistic support but limited and inconsistent human evidence (IARC).
How to reduce your risk
Limit long-term night-shift exposure when possible. Prioritize healthy habits like exercise, a balanced diet, quality sleep during off hours, and routine medical screenings. Employers may help by providing appropriate lighting, ergonomic scheduling, and sleep hygiene education.
Bottom line
Night shift work is probably cancer-causing, according to IARC. Studies show modest increases in breast cancer risk for short-term and long-term night shift work, though findings vary across studies. While uncertainty remains, circadian disruption is likely relevant, and minimizing long-term exposure may help reduce risk.