A cheap, over-the-counter sleep hormone may do far more for the roughly one in five workers who labor through the night than simply help them fall asleep during daylight hours. A small clinical trial published in Occupational & Environmental Medicine — and brought to renewed public attention by ScienceDaily on May 30, 2026 — found that night shift workers who took a 3 mg melatonin supplement before daytime sleep showed substantially higher levels of a validated DNA repair biomarker compared with those who took a placebo. The finding matters because chronic night shift work is classified by the International Agency for Research on Cancer as a probable human carcinogen, and impaired DNA repair is one of the leading biological explanations for why.
Why Night Shift Work Damages DNA
For nurses, paramedics, freight handlers, factory operators, and security workers — a population that accounts for roughly 20% of the global workforce, or an estimated 700 million people — circadian disruption is an occupational constant. When the body is forced to stay awake through the night, its natural production of melatonin is suppressed by artificial light exposure. Melatonin is produced by the pineal gland in the brain and rises naturally in darkness; for night shift workers, those hours of darkness coincide with working time, so the hormone never gets the signal to rise.
That suppression has downstream consequences. The body's cells are continuously exposed to reactive oxygen species — chemically unstable molecules generated by normal metabolism — which oxidize DNA bases and create lesions that, if left unrepaired, can accumulate and eventually drive the mutations associated with cancer. Melatonin has known antioxidant properties and plays a role in coordinating the body's repair processes during sleep. When its production is blunted for years, the body's capacity to clear this oxidative damage appears to diminish. The International Agency for Research on Cancer has classified night shift work as Group 2A — "probably carcinogenic to humans" — a designation it first issued in 2007 and reconfirmed in 2019 after reviewing more than a decade of additional evidence.
What the Trial Measured — and What It Found
A team led by Umaimah Zanif and Dr. Parveen Bhatti at the BC Cancer Research Institute and the University of British Columbia conducted the first randomized, placebo-controlled trial specifically designed to test whether melatonin supplementation could improve oxidative DNA repair capacity in night shift workers.
The study enrolled 40 participants between the ages of 18 and 50 who had worked at least two consecutive night shifts per week, lasting at least seven hours each, for a minimum of six months. None had sleep disorders or chronic health conditions. Half were assigned to take a 3 mg melatonin pill with food, approximately one hour before their daytime sleep after a night shift, for four weeks. The other half took an identical placebo on the same schedule. Participants wore activity trackers to monitor actual sleep duration.
The researchers measured urinary levels of 8-hydroxy-2′-deoxyguanosine, known as 8-OHdG — a well-established biomarker of oxidative DNA damage repair capacity. When the body's cellular machinery identifies and excises oxidized DNA bases, the repaired fragments are excreted in urine; higher urinary concentrations of 8-OHdG indicate more active repair. Among those taking melatonin, urinary 8-OHdG was 80% higher during daytime sleep compared with the placebo group — a 1.8-fold increase. This finding was described by the authors as borderline statistically significant, with a p-value of 0.06, meaning it fell just outside the conventional threshold for statistical significance and must be interpreted with caution.
Notably, the effect did not carry over to the subsequent night shift: no significant difference in 8-OHdG levels was observed between the two groups during the work period itself. The benefit appeared limited to the sleep phase when melatonin was taken.
How Does Melatonin Boost DNA Repair During Sleep?
When oxidized DNA bases accumulate — particularly 8-oxoguanine, one of the most common lesions caused by reactive oxygen species — the body's cellular machinery removes them through a repair process, releasing the excised fragments into the bloodstream and ultimately into urine. Melatonin appears to coordinate this process during sleep. The researchers propose that by restoring a melatonin signal during the daytime sleep period following a night shift, the supplement helped reinstate part of the circadian coordination of repair that disrupted schedules suppress.
The study measured the repair biomarker, not the underlying repair pathway directly, so the precise molecular mechanism by which melatonin enhanced the 8-OHdG signal during daytime sleep requires further investigation. What the trial established is that a measurable, repair-associated biomarker responded to melatonin in the context of real-world night shift work — for the first time in a controlled clinical setting.
How Does Night Shift Work Raise Cancer Risk?
The link between night shift work and cancer does not hinge on a single mechanism. The International Agency for Research on Cancer's 2019 review cited circadian disruption, altered hormone signaling, changes in immune function, metabolic disruption, and impaired DNA repair as contributing pathways. The DNA repair pathway is particularly significant because it operates continuously — the body is always generating and clearing oxidized DNA bases as a byproduct of normal cellular processes, and any chronic impairment of that clearance creates a slow accumulation of unrepaired genomic damage over years and decades.
The Canadian Cancer Society, which helped fund Dr. Bhatti's research through a 2022 Action Grant, characterized the findings as opening a potential new avenue for cancer prevention in night shift workers. Dr. Bhatti described impaired oxidative DNA damage repair as "a compelling mechanism that may contribute to the carcinogenicity of night shift work" and noted that the trial results "warrant future larger-scale studies."
Significant Limitations Require Honest Assessment
The researchers are explicit about what this trial cannot establish. With only 40 participants, the study was not large enough to draw broad conclusions. Most participants worked in healthcare, which means the results may not generalize to night shift workers in manufacturing, logistics, or emergency services. The borderline statistical significance of the primary finding means chance cannot be ruled out as an explanation for the difference observed. The trial also could not control for natural light exposure outside of work hours, which affects circulating melatonin levels independently of supplementation.
Most critically, the study measured a biomarker associated with DNA repair activity — not cancer incidence, not cancer risk, and not actual tumor formation. The gap between a shift in a urinary biomarker and a meaningful reduction in cancer rates over a working lifetime is vast and would require large, long-duration intervention trials to close.
Safety Is Not a Given: What Long-Term Melatonin Use May Cost
For night shift workers considering melatonin, a parallel finding from November 2025 deserves careful attention. Researchers presenting at the American Heart Association's Scientific Sessions 2025 reported that, among more than 130,000 adults with insomnia who had used melatonin for at least one year, the supplement was associated with a 90% higher risk of heart failure diagnosis, increased rates of heart failure hospitalization, and higher all-cause mortality — compared with matched non-users who did not take melatonin. That study is also preliminary and cannot establish a causal relationship, but the scale of the dataset makes the signal worth noting.
In the United States, melatonin is sold over the counter as an unregulated dietary supplement; it is not approved by the Food and Drug Administration for any medical condition. In the United Kingdom, it is a prescription-only medication. These regulatory differences reflect legitimate uncertainty about the appropriate conditions for use. The BC Cancer trial used a 3 mg dose — a common over-the-counter strength — but some products contain significantly more, and actual content has been found to vary widely from label claims.
What Researchers Say Should Come Next
The BC Cancer team called explicitly for larger trials examining different doses, longer supplementation periods, and more diverse populations. Dr. Bhatti noted that anyone working nights for many years would need to take melatonin consistently over that same period to maximize any potential protective effect — a prolonged exposure that underscores the importance of understanding long-term safety before any recommendation can be made.
Current options for managing the health burden of shift work remain largely behavioral: careful scheduling of sleep windows, controlled light exposure therapy, and dietary adjustments. A pharmacological intervention as accessible as melatonin, if validated in larger trials, would represent a meaningful addition. For the hundreds of millions of workers whose health is shaped by what time of day their job requires them to be awake, the question is worth pursuing rigorously.
For now, the practical takeaway is not a prescription but a conversation: night shift workers who are concerned about the cumulative health effects of their schedule may have grounds to ask an occupational health physician about melatonin as part of a broader health management plan — with the understanding that the evidence base is still at an early stage and that long-term safety questions remain open.
Frequently Asked Questions
Does melatonin reduce the cancer risk of night shift workers?
A small clinical trial published in Occupational & Environmental Medicine found that 3 mg of melatonin before daytime sleep raised a DNA repair biomarker by 80% in night shift workers compared with placebo, suggesting it may support one of the biological mechanisms thought to link shift work to cancer risk. However, the study did not measure cancer incidence and its primary finding was borderline statistically significant, so larger trials are needed before any protective effect on cancer risk can be confirmed.
How much melatonin should night shift workers take?
The randomized trial used a dose of 3 mg, taken with food approximately one hour before daytime sleep. This is within the commonly available over-the-counter range in the United States and Canada, though melatonin is a prescription-only drug in the United Kingdom. Researchers have not yet determined whether a higher or lower dose would be more effective or safer, and the lead team has called for larger studies testing varying doses.
Why do night shift workers have a higher cancer risk?
The International Agency for Research on Cancer classifies night shift work as Group 2A — probably carcinogenic to humans — based on evidence linking it to elevated rates of breast, prostate, and colorectal cancer. The leading biological explanations include suppressed melatonin production due to nighttime light exposure, impaired DNA repair capacity, disrupted hormone signaling, immune function changes, and metabolic disruption — all downstream effects of chronic misalignment between the body's internal clock and the external work schedule.
Is melatonin safe to take every night long-term?
Safety data for long-term melatonin use remain limited. A preliminary study presented at the American Heart Association's Scientific Sessions 2025, drawing on records from more than 130,000 adults with insomnia, found that those who took melatonin for at least one year had a significantly higher risk of heart failure and all-cause mortality compared with matched non-users. That association has not been confirmed as causal, but it is large enough to warrant caution. The U.S. Food and Drug Administration does not regulate melatonin as a drug, and product quality varies significantly across brands.
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