Melatonin As Cancer Fighter? Maaaaybe…

The last time I was researching the link between cancer and sleep, noting the myriad benefits gained from solid nighttime rest, I was surprised to see mention of melatonin’s role in decreasing the risk of cancer.

For anyone who might not be familiar with it, melatonin (a tryptophan derivative) is a naturally-occuring hormone secreted by the pineal gland that signals when it’s time to sleep and wake. It’s mediated by light levels, with the amount of melatonin in your body increasing as the sun goes down. You’ve probably seen melatonin on the vitamin shelves at your local store, as in recent years it’s been popularized as a non-addictive sleep aid. What I hadn’t realized was that its effect on cancer cells has become an active area of study.

I dug into the PubMed database to find there was quite a bit on this topic. However, note that not all the journals in which these results were published were familiar to me, so I cannot vouch for the rigor of the peer review, however, there was a general consensus that melatonin showed promise.

Melatonin shows a lot of promise as a cancer fighting hormone.

It’s well-established that women who work night shifts experience disruption of their circadian cycle and have an increased risk of breast cancer risk, purported to result from extra circulating estrogen (Cohen et al., 1978, Lancet). Researchers are now linking that disruption with a decrease in melatonin production.

Amin et al. (2019, J Cell Biochem) describe the action of melatonin as it relates to cancer: “Melatonin via its receptors and various second messenger pathways decrease[s] cell duplication and increase[s] cell differentiation.” Since cancer tumors are composed of a proliferation of poorly differentiated cells, this means that the action of melatonin works against the process by which cancer develops and progresses.

Amin et al. continue by noting that melatonin “regulates estrogen-dependent pathways (by nonreceptor-dependent means) and reduces the production of oxidants; as a result, melatonin inhibits cell toxicity and mutations….Melatonin interrupts estrogen-dependent cell signaling and also causes reduced estrogen-stimulated cells in breast cancer. [It] is a mammary tumor inhibitor…[as relates to the] development, progression, and metastasis of breast cancer via a number of molecular mechanisms.”

A randomized, double-blind, placebo-controlled research study showed that melatonin has a neuroprotective effect that can counteract the effects of chemotherapy on “cognitive function, sleep quality and depressive symptoms” (Palmer et al., 2020, PLOS One). These are significant side effects that have a profound impact on the patient’s quality of life, and anything that may relieve these will improve the entire treatment experience.

Griffin & Marignol (2018, Int J Radiat Biol) noted that melatonin administered to subjects before they were exposed to ionizing radiation resulted in the breast cancer cells being more sensitized to the radiation therapy, rendering it more effective. And melatonin seemed to reduce the radiation-induced side effects exhibited by both human and rodent subjects.

No matter how many drug treatments are available for cancer, they do no good if the cancer cells develop a resistance to them. In a study published this year, Sang et al. (2021, Cancer Lett) found that melatonin increased the effectivess of drug lapatinib in HER2 receptor-positive breast cancer cells that were originally resistant to the drug, suggesting that melatonin could be a promising adjuvant therapy for treating advanced HER2+ tumors.

So, melatonin may reduce breast cancer risk, make existing treatments more effective and help protect patients against negative effects of these therapies. Does that mean you should run out and gobble melatonin every night?

Many studies are first run on animal subjects, but to truly determine whether a treatment will be effective for cancer patients, it must be tested on humans.

No! As tempting as it sounds, that’s not an advisable course of action. Many more studies still have to be run to evaluate the exact mechanisms by which melatonin acts on physiological processes. Some of the results in the cited studies were based on small sample sizes; good for proof of concept, but following up with larger scale studies is critical. Some studies were run on animal models which are not the best human analogues. In addition, there’s little direction regarding proper therapeutic dosages. Establishing those will take additional research.

Keep in mind: a naturally-occurring hormone like melatonin likely has a “sweet spot” in terms of dosing, and determining the ideal amount may be tricky. Just because you can buy melatonin gummies in 10mg doses does not mean you should be taking that much.

Furthermore, melatonin may elicit negative side effects in some people, including headaches, nightmares and nausea. Side effects tend to be short-lived with short-term usage but there’s still not enough information available about long-term safety, so taking it for longer periods of time is strongly discouraged.

Note also, the articles I’ve mentioned above were selected because they describe recent research, although some of these are review articles that espouse the authors’ opinion, backed up by research selected for the purpose. If you’d like to read the above studies yourself and the links I’ve posted do not provide you full access, please consult your local university library for copies (copyright laws prohibit me from providing access to pay-only articles, regrettably).

Finally, it may be that some of melatonin’s benefits might be its undoing. Reiter et al. (2017, Int J Mol Sci) note that melatonin is inexpensive and readily available, and therefore there might not be the same level of interest in researching and developing it for cancer use as there might be with a novel drug with the potential to be more lucrative.

Where does this leave us?

I would urge you to:
1) Ask your oncologist about what they would recommend, given the research that’s coming out. They are still your best source for information. is only a blog and can be used as food-for-thought but definitely not for determining your course of treatment.
2) Learn how to read scientific studies. There are free courses on educational site such as that explain research design and interpretation in layperson terms. They can offer instruction on reading research with a critical eye.
3) Keep an eye on emerging research. Databases such as PubMed are excellent sources for health research. Even if you’re not well-versed in research design, you can look up articles to bring to your next visit with a health provider.
4) Do not take megadoses of melatonin! There is still so much we have to learn about this hormone as it relates to cancer, and self-medicating with melatonin in the hopes that “maybe it’ll help” is dangerous. Again, your oncologist remains your best source of information.

Promising drugs aside, get your sleep!

I do encourage you to respect your circadian rhythm by establishing good sleep hygiene practices to improve the conditions for your body to create and release its own melatonin. Proper and adequate sleep will always benefit you!

And so we get back to the idea that launched this post: sleep remains the ultimate good.


It bears repeating: ALWAYS ask your cancer team about starting any new medication or supplement, regardless of how well-supported it is by research.

Sleep: Still the Ultimate Good

Some time back I posted sleep researcher Dr. Matt Kelly’s TED Talk on sleep, which I highly recommend. Now, the American Academy of Sleep Medicine (AASM) has issued a position paper which states unequivocally that “sleep is essential to health”.

As noted in the position paper, “Healthy sleep is important for cognitive functioning, mood, mental health, and cardiovascular, cerebrovascular, and metabolic health.” Anyone who has suffered through the daylight savings time change – that would be most of us – can tell you that even missing out on a single hour of sleep can leave you feeling off for a few days.

Sleeplessness is too often an occurence for too many of us.

And many of us have likely had the experience of sacrificing sleep for school or work projects, adjusting to newborns, or other similar temporary situations, not to mention the occasional middle-of-the-night stress session.

But sleep disruption on a chronic scale has far-reaching repercussions, and has been “associated with an increased risk of mortality and contributes to both the individual risk and societal burden associated with several medical epidemics, including cardiovascular disease, diabetes, obesity, and cancer.”

In particular, shift work has been associated with an increased risk of cancer, due to the persistent disruption of the body’s biological clock, as reported in a recent CDC blog post based on reviews by the National Toxicology Program and the International Agency for Research on Cancer. Data came from both breast and prostate cancer surveys. [The CDC post does outline what shift workers can do to protect themselves.]

I suspect that data from shift workers will reveal the influence of factors other than simply how long one sleeps, for instance, the significant effect of melatonin’s known anticancer properties (Hill et al., 2015, Endocr Relat Cancer; Yi et al., 2017, Oncotarget; Bondy & Campbell, 2018, Int J Mol Sci).

While the studies may be confusing, good sleep hygiene is always important.

According to the AASM position paper, the consensus among different sleep-related organizations is that adults require at least 7 hours of sleep regularly, with the range being 7-9 hours. While this seems straightforward, one meta-analysis (Lu et al., 2017, Biomed Res Int) of 10 studies suggested that sleeping excessive amounts (over 9 hours) was linked to an increased risk of estrogen-positive (but not estrogen-negative) breast cancer for women. At the same time, Xiao et al. (2016, Sleep Med) found that short sleep duration was associated with triple-negative (but not estrogen-positive) breast cancer in black women as compared to white women, suggesting racial disparities in the data, so further research is needed.

Confusing? Yes, this is clearly an area that calls for more study. Some clarification came recently from the publication of the “Million Woman Study”. This was an extensive prospective multi-year study of women in the UK that, you guessed it, found no association between breast cancer and sleep of any duration (Wong et al., 2021, Sleep). Nonetheless, the authors did note some shortcomings of the study, so this question is likely to be revisited.

Take home message? Conflicting studies aside, everyone would agree that good sleep hygiene (see CDC recommendations) is important no matter what your cancer risk. With our lives running 24/7 and sleep schedules constantly being disrupted, we should take a clue from the animals who settle down as the evening begins, like clockwork. We might have advanced as a civilization, but we can’t get past the reality that when it’s dark out, it’s time to hit the sack.