VDC test kit slider

Does working the night shift increase your risk of internal cancers?

Posted on: April 25, 2013   by  Dr William Grant


For over a decade, it has been acknowledged that those who work night shifts have an increased risk of developing internal cancers.1 From that paper until the present time, the primary hypothesis to explain the finding was disruption of the circadian rhythm and reduced production of melatonin. The evidence for an effect of melatonin on reducing risk of cancer is strongest for breast cancer.2 For example; breast cancer has peaks of diagnosis in spring and fall.3 The hypothesis to explain this finding is that vitamin D reduces risk of breast cancer in summer while melatonin associated with reduced sunlight in winter reduces breast cancer risk in winter. However, there does not seem to be such seasonality for other types of cancer.

A recent study from Canada found that men who worked night shifts had increased incidence rates of cancers of the bladder, colon, lung, pancreas, and prostate, and non-Hodgkin’s lymphoma.4 They found non-significant increased incidence for cancers of the esophagus, kidney, and stomach. They did not find that cancer risk was related to cumulative duration of night work. One wonders whether night shift workers in Canada tend to be night owls.

The evidence that solar ultraviolet-B (UVB) and vitamin D reduce the risk of cancers of the bladder, colon, kidney, lung, pancreas, and stomach, and non-Hodgkin’s lymphoma is relatively strong.5, 6 There is evidence that UVB reduces risk of prostate cancer, but no evidence that vitamin D reduces risk of prostate cancer.7 While the authors in their response note that farmers have an increased risk of prostate cancer, they omitted to cite their paper reporting that exposure to pesticides was an important risk factor for prostate cancer among farmers in Canada.8 My recent study of occupation and cancer incidence in Nordic countries found a modest inverse correlation for the solar UVB index associated with outdoors work and incidence of prostate cancer.6 The discrepancy between ecological studies showing reduced risk of prostate cancer associated with solar UVB and the lack of an association between serum 25-hydroxyvitamin D and prostate cancer incidence rates is puzzling.9

A study among female nurses in the United States found that night shift work for women was associated with reduced risk of melanoma and non-melanoma skin cancer.10 Risk was reduced in proportion of years of night shift work, which is a dose-dependent relation. It was also reported that those with dark brown/black hair color had significant dose-dependent reductions in all three types of skin cancer but those with light brown or red/blonde hair did not. This finding can be understood in that those with light colored hair have lighter skin and are more likely to burn in the sun, so try to avoid sun exposure.11 The authors also reported no effect that could be attributed to sun exposure in terms of residence, ultraviolet light exposure level at baseline, or location of skin cancer. While they admitted that the finding of reduced skin cancer incidence rates was in contrast to many previous studies reporting increased risk of internal cancers associated with night shift work, they proposed that there was a possible genetic component that might explain the extent of melatonin suppression for women with dark hair. In my opinion, this study strongly supports the hypothesis that night shift workers spend less time in the sun than others.

In my letter to the editor, I stated that the findings from the Canadian and U.S. studies were consistent with low solar UVB irradiance due to sleeping during the day.12

In further discussions with those who study night shift work, it was pointed out that a number of factors could explain the increased risk of internal cancers among night shift workers including low melatonin and adverse diets due to eating from vending machines.

The authors in their response pointed out that Montreal, where the study was conducted, was at 45º N, and it is impossible to produce vitamin D from solar UVB for about half of the year. In response to that point, in my study of cancer incidence rates among people in the five Nordic countries with a latitude range of 55º-70º N, there is significantly reduced risk of many types of cancer for those with the occupations with the greatest amount of time spent out of doors.6

While other risk factors for cancer among night shift workers cannot be ruled out, the easiest one to correct and the one with strong evidence is vitamin D deficiency. Based on numerous studies, night shift workers should consider taking 2000-5000 IU/d vitamin D3 and raising serum 25-hydroxyvitamin D levels to 40-60 ng/ml.13, 14


  1. Tynes T, Hannevik M, Andersen A, Vistnes AI, Haldorsen T. Incidence of breast cancer in Norwegian female radio and telegraph operators. Cancer Causes Control. 1996 Mar;7(2):197-204.
  2. Sanchez-Barcelo EJ, Mediavilla MD, Alonso-Gonzalez C, Rueda N. Breast cancer therapy based on melatonin. Recent Pat Endocr Metab Immune Drug Discov. 2012 May;6(2):108-16.
  3. Oh EY, Ansell C, Nawaz H, Yang CH, Wood PA, Hrushesky WJ. Global breast cancer seasonality. Breast Cancer Res Treat. 2010 Aug;123(1):233-43.
  4. Parent ME, El-Zein M, Rousseau MC, et al. (2012) Night work and the risk of cancer among men. Am J Epidemiol. 2012;176(9):751–759.
  5. Grant WB. (2012) Ecological studies of the UVB–vitamin D–cancer hypothesis; review. Anticancer Res. 2012;32(1):223-36.
  6. Grant WB. (2012) Role of solar UV irradiance and smoking in cancer as inferred from cancer incidence rates by occupation in Nordic countries. Dermatoendocrinol. 2012;4(2):203-11.
  7. Grant WB. (2010) Relation between prediagnostic serum 25-hydroxyvitamin D level and incidence of breast, colorectal, and other cancers. J Photochem Photobiol B, 2010;101(2):130–6.
  8. Parent ME, Désy M, Siemiatycki J. (2010) Does exposure to agricultural chemicals increase the risk of prostate cancer among farmers? Mcgill J Med. 2009 Jan;12(1):70-7.
  9. Schwartz GG. Vitamin d, sunlight, and the epidemiology of prostate cancer. Anticancer Agents Med Chem. 2013 Jan 1;13(1):45-57.
  10. Schernhammer ES, Razavi P, Li TY, Qureshi AA, Han J. Rotating night shifts and risk of skin cancer in the nurses’ health study. J Natl Cancer Inst. 2011;103(7):602–606.
  11. Glass D, Lens M, Swaminathan R, Spector TD, Bataille V. Pigmentation and vitamin D metabolism in Caucasians: low vitamin D serum levels in fair skin types in the UK. PLoS One. 2009 Aug 3;4(8):e6477.
  12. Grant WB. (2013) Re: “Night work and the risk of cancer among men”; author reply. Am J Epidemiol. 16 Apr 2013 epub.
  13. Grant WB. (2011) An estimate of the global reduction in mortality rates through doubling vitamin D levels. Eur J Clin Nutr. 2011 September;65(9):1016-26.
  14. Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad MH, Weaver CM. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 2011 Jul;96(7):1911-30.

2 Responses to Does working the night shift increase your risk of internal cancers?

  1. Rita and Misty

    Greetings Dr. Grant,

    Thank you for a very interesting article.

    I’d like to comment on the following quote from your blog: “In further discussions with those who study night shift work, it was pointed out that a number of factors could explain the increased risk of internal cancers among night shift workers including low melatonin and adverse diets due to eating from vending machines.”

    We all know that food from vending machines is typically garbage, so I certainly agree with those who study night shift work on at least this point. However, regarding low melatonin levels, don’t the researchers realize that low melatonin is very much related to sun exposure..and dare I say vitamin d levels? Of course melatonin would be tied into increased risk of internal cancers…right along with low 25(OH)D levels and lack of adequate sun exposure.

    (I’m not a physician or scientific researcher so I do apologize if I am oversimplifying things just a bit…and I do hope you will inform me if I am missing something significant here…how else would I learn?)

    VDC members will know that I so often return to one of my all time favorite journal articles:Benefits of Sunlight: A Bright Spot for Human Health–but it is such an informative article! I do hope VDC members will take the time to read it.


    Here is some great information on Melatonin taken directly from the linked article:

    Serotonin, Melatonin, and Daylight

    As diurnal creatures, we humans are programmed to be outdoors while the sun is shining and home in bed at night. This is why melatonin is produced during the dark hours and stops upon optic exposure to daylight. This pineal hormone is a key pacesetter for many of the body’s circadian rhythms. It also plays an important role in countering infection, inflammation, cancer, and auto-immunity, according to a review in the May 2006 issue of Current Opinion in Investigational Drugs. Finally, melatonin suppresses UVR-induced skin damage, according to research in the July 2005 issue of Endocrine.

    When people are exposed to sunlight or very bright artificial light in the morning, their nocturnal melatonin production occurs sooner, and they enter into sleep more easily at night. Melatonin production also shows a seasonal variation relative to the availability of light, with the hormone produced for a longer period in the winter than in the summer. The melatonin rhythm phase advancement caused by exposure to bright morning light has been effective against insomnia, premenstrual syndrome, and seasonal affective disorder (SAD).

    The melatonin precursor, serotonin, is also affected by exposure to daylight. Normally produced during the day, serotonin is only converted to melatonin in darkness. Whereas high melatonin levels correspond to long nights and short days, high serotonin levels in the presence of melatonin reflect short nights and long days (i.e., longer UVR exposure). Moderately high serotonin levels result in more positive moods and a calm yet focused mental outlook. Indeed, SAD has been linked with low serotonin levels during the day as well as with a phase delay in nighttime melatonin production. It was recently found that mammalian skin can produce serotonin and transform it into melatonin, and that many types of skin cells express receptors for both serotonin and melatonin.

    With our modern-day penchant for indoor activity and staying up well past dusk, nocturnal melatonin production is typically far from robust. “The light we get from being outside on a summer day can be a thousand times brighter than we’re ever likely to experience indoors,” says melatonin researcher Russel J. Reiter of the University of Texas Health Science Center. “For this reason, it’s important that people who work indoors get outside periodically, and moreover that we all try to sleep in total darkness. This can have a major impact on melatonin rhythms and can result in improvements in mood, energy, and sleep quality.”

    For people in jobs in which sunlight exposure is limited, full-spectrum lighting may be helpful. Sunglasses may further limit the eyes’ access to full sunlight, thereby altering melatonin rhythms. Going shades-free in the daylight, even for just 10–15 minutes, could confer significant health benefits.

  2. Rita and Misty

    I’d like to emphasis that it may very well be necessary to leave sunglasses off for 10-15 minutes to maximize sunlight’s benefits regarding serotonin and melatonin. Quoted from my above-linked article (for those who might have missed it) : “Sunglasses may further limit the eyes’ access to full sunlight, thereby altering melatonin rhythms. Going shades-free in the daylight, even for just 10–15 minutes, could confer significant health benefits.”

Test Your Vitamin D Levels at Home!

Our in-home vitamin D test kit is easy, affordable, and an accurate way to find out your Vitamin D status.

order NOW

We need your help!

We're spreading awareness on Vitamin D Deficiency
Donate NOW
Latest Articles
4 tips for writing an essay in English

Writing an essay in English can be a challenge, especially if you are beginning to learn the language or do not yet fully master it. But don’t worry, did you know that there are strategies that can make this task easier for you? To help you meet this challenge, we leave you a series of […]

Weekly Newsletter

Our Sponsors

December 21st is DDAY. Click here to celebrate the day with us!