What melatonin is (quick)

Melatonin is a hormone made by the pineal gland that helps control sleep/wake cycles (circadian rhythm). It also has antioxidant, immune-modulating and regulatory effects in cells — features that are why researchers study it in cancer. PMC

How melatonin might affect cancer (mechanisms)

Researchers propose multiple ways melatonin could influence cancer biology:

Antioxidant and (reduces ). PMC

Direct anti-tumor actions: slowing cancer cell proliferation, encouraging programmed cell death (apoptosis), and reducing metastasis/angiogenesis in lab studies. PMC

Modulating immune responses (may boost anti-tumour immunity). PMC

Restoring circadian rhythm (disrupted rhythms are linked to higher risk for some cancers, e.g., in long-term night shift workers). MDPI

1. Direct Anti-Cancer Mechanisms

Melatonin is thought to directly target cancer cells through multiple pathways

Inhibiting Cell Proliferation: Melatonin can interfere with the cell cycle, which is the process cells use to grow and divide.⁴ By arresting the cycle, particularly in the G2/M phase, it can hinder the rapid expansion of malignant cells.
Inducing Apoptosis (Programmed Cell Death): It can promote the self-destruction of cancer cells, often by disrupting mitochondrial function and activating pro-apoptotic proteins like caspases.⁶ Interestingly, this action often appears to be selective, promoting apoptosis in cancer cells while protecting normal cells.
Antioxidant Activity: Melatonin is a powerful antioxidant and free-radical scavenger. By protecting cellular components, including DNA, from oxidative damage, it may help prevent the initial stages of carcinogenesis.
Anti-Angiogenesis: It can inhibit the formation of new blood vessels (angiogenesis) that tumors need to grow and spread.
Inhibiting Metastasis: Melatonin has been shown to suppress the migration and invasion of cancer cells, which is key to preventing the spread of cancer to distant sites.

2. Modulation of Hormone-Dependent Cancers

Melatonin is especially relevant in hormone-dependent cancers like breast cancer and prostate cancer:

Anti-Estrogenic Effects (Breast Cancer): Melatonin can act as an anti-estrogen by reducing the expression of the estrogen receptor alpha ( $ER\alpha$ ) and inhibiting the binding of estrogen to its receptors. This reduces the growth-stimulating signal that estrogen provides to some breast cancer cells.

3. Impact on Standard Cancer Treatment

Melatonin is being studied as an adjuvant therapy (used alongside standard treatments) due to its potential to:

Enhance Efficacy: It may increase the sensitivity of cancer cells to chemotherapy and radiotherapy, potentially making these treatments more effective.
Reduce Side Effects: It may help mitigate some of the toxic side effects of chemotherapy and radiation, such as fatigue, nausea, and damage to healthy cells, thereby improving a patient's quality of life.

4. Links to Circadian Rhythm

The natural production of melatonin is linked to the body's circadian rhythm (the 24-hour cycle).

Disrupted Rhythms and Risk: Studies, particularly in night shift workers, have suggested an association between chronic disruption of the normal light-dark cycle (leading to lower nighttime melatonin levels) and an increased risk for certain cancers, especially breast and prostate cancer.¹⁸ The theory is that the physiological surge of melatonin at night is a "natural restraint" on tumor development.

What the human studies say (short version)

Preclinical (cells/animals): Many studies show promising anti-cancer effects. PMC
Clinical trials / meta-analyses: Results are mixed but interesting. Some meta-analyses and small randomized trials report improvements in short-term outcomes (for example better 1-year survival and reduced chemotherapy/radiation side effects) when melatonin was used as an adjuvant (added to standard treatment). Other systematic reviews find little or no benefit for quality of life or longer-term outcomes — largely because trials are small, heterogeneous (different cancers, doses, timings), and of variable quality. In short: there are suggestive benefits in some trials, but the evidence is not strong enough yet to call melatonin a proven anti-cancer treatment. MDPI+1

Doses used in studies (what researchers have tried)

Clinical studies have used a wide range, commonly anywhere from about 3 mg up to 20 mg nightly, sometimes higher in specific short courses during chemotherapy. Different doses and formulations were used in different trials, so there’s no single “standard cancer dose.” (Trials often used higher doses than typical over-the-counter sleep doses.) ScienceDirect+1

Safety and drug interactions (important)

Short-term use of melatonin is generally well-tolerated for most people; common side effects include drowsiness, headache, and occasionally vivid dreams. Long-term safety is less certain. Drugs.com+1
Interactions: melatonin can interact with other medications (for example, anticoagulants like warfarin, some blood pressure drugs, CYP1A2 substrates, and possibly some immunosuppressants). Because cancer patients often take many medicines (chemotherapy, targeted drugs, steroids, anticoagulants), interactions are a real concern. Memorial Sloan Kettering Cancer Center+1
Effect on cancer treatment: Some lab studies suggest melatonin may increase sensitivity to chemo/radiotherapy and reduce side effects, but because of drug-interaction and timing issues, it must only be used after talking with the oncology team. PMC

Practical takeaways (what this means for someone)

Melatonin is not a proven cancer cure. It’s being researched as a possible supportive (adjuvant) agent and for improving sleep/side effects, but the evidence is not definitive. MDPI+1
It can help sleep and may reduce some treatment side effects in some patients, according to several trials, which is useful because better sleep can improve quality of life. ScienceDirect+1
Consult with your oncologist or pharmacist before using melatonin. Because of possible interactions with cancer drugs and other medicines (and because dosages in trials vary), a doctor should advise whether it’s safe and when/how to take it. Memorial Sloan Kettering Cancer Center+1
If someone is working nights or exposed to light at night, reducing light exposure in the evening (blue light blocking, dark sleeping environment, consistent schedule) is a safe way to support natural melatonin and circadian health — and that may be relevant for cancer prevention strategies.