Human studies include reduced rates of cancer amongst totally blind people who have melatonin all the time. As well as studies looking at reduced cancer rates in less industrialised countries who have less exposure to light at night.

There are many animal studies looking at the effect of continuous light on animals and also melatonin on tumour growth.

 Human Studies

1978 M Cohen gives five reasons why reduced pineal secretion increase the risk of cancer:

  1. Pineal calcification is most common in countries with high rates of breast cancer.
  2. Psychiatric patients taking Chlorpromazine, which raises serum melatonin, have a lower incidence of breast cancer.
  3. The pineal and melatonin may influence tumour induction and growth in experimental animals.
  4. Melatonin receptors found in the human ovary suggest a direct influence of melatonin on ovarian function and possibly oestrogen production.
  5. Impaired pineal secretion is believed  to be an important factor in triggering early puberty. Early menarche is a risk factor for breast cancer.

1989 Barni S et al found that cancer patients with the best prognosis had the highest melatonin concentration. Mean levels were significantly higher in estrogen receptor positive patients. Concluded that Melatonin plays a role in hormone dependency of human breast cancer.

1989 Lissoni P et al Began trying Melatonin with other treatments on humans based on animal studies.

1991 Hahn RA found the incidence of breast cancer in totally blind women was about half that of sighted women.

1992 Stevens et al suggests the high rate of breast cancer in industrialised countries is due to the use of electricity to produce light at night or electromagnet fields themselves, compared to non industrialised societies.

1996 Stevens presents ‘Melatonin Hypothesis’ stating Light at Night LAN and EMF suppress melatonin and cause a high incidence of breast cancer.

1998 A study by Feychting M et al in Sweden showed totally blind people had approx 2/3 risk of cancer of all types in both men and women, when compared to sighted people.

1999 Kukala et al in Finland found totally blind men had twice the rate of all cancers compared to society in general. The higher smoking rate amongst blind people was one explanation. Another study found lower breast cancer among women.

1999 Erren reported uniformly low risk for hormone dependent cancer in the artic.

1999 Brainard G reviewed all the evidence that light at night and subsequent melatonin suppression might be a source of elevated cancer rate in advanced societies- concluded more studies should be done and is a foremost research in the field.

2000 Bartsch H found Melatonin inhibited the growth of some but not all ovarian and breastcancer cultures. But he found that pineal extract inhibited the growth of all of them. There appear to be other hormones in the pineal that may be more powerful anticancer agents than melatonin.

2001 Kliukliene et al. Found blind women had significantly lower breast cancer rate (15,000 women).

2001 Two milestone papers suggest specific blue component of light was responsible for the supression of melatonin.

2001 Czeisler found blind and sighted individuals produced Melatonin for 9 -11 hours if the sighted inividuals were kept in darkness. Suggesting that being in the dark for more than 11 hours will not result in further increase in melatonin.

2001 Japanese paper described a study where patients with delayed sleep phase syndrome showed significantly greater suppression of Melatonin when given exposure to light at night than controls. They suggested they may have a great risk of cancer.

2002 Glickman states under highly controlled exposure circumstances less than 1 lux of monchromatic light elicited a significant suppression of melatonin.

2002 At The International symposium on light, endocrine and cancer – Poole said he expected growth in scope and scale of studies

2002 Stevens 5 fold difference between cancer in primitive societies and indurstrialised.

2002 Anisimov – “The role of the modulation of the pineal gland function in development of cancer is discussed in the review. An inhibition of the pineal function with pinealectomy or with the exposure to the constant light regimen stimulates mammary carcinogenesis, whereas the light deprivation inhibits the carcinogenesis. Epidemiological observations on increased risk of breast cancer in night shift workers, flight attendants, radio and telegraph operators and on decreased risk in blind women are in accordance with the results of experiments in rodents. Treatment with pineal indole hormone melatonin inhibits carcinogenesis in pinealectomized rats or animals kept at the standard light/dark regimen (LD) or at the constant illumination (LL) regimen.”

2003 Blask Brainard et al describe growth of human cancer cells transplanted into rats under constant light melatonin was minimised and tumours grew rapidly. Alternating light dark allowed Melatonin cucle tumours grew only slowly. This is the first biological evidence of a link between constant light exposure and increased human oncogenesis involving melatonin suppression.

2003 Anisimov adds that Pineal peptide preparations Epithalamin and synthetic tetrapepetide Epitalon are potent inhibitors of carcinogenesis in rodents.

2003 Schernhammer et al Found working rotating nightshift work for many years increases the risk of colorectal cancer and breast cancer.

2003 Filipski et al found that tumour growth in mice with lesioned Super chaismatic nucleiwas faster than controls in both glasgow osteosarcoma and pancreatic adenocarcinoma.

2004 Pauley states that lighting has become a public health issue.

2004 Filipski found tumours grew faster in jet-lagged animals as compared to controls where as exposure to constant light or constant dark had no effect. Jet lag was advancing light dark cycle by 8 hours every 2 days.

2005 Verkasalo PK showed women who consistently sleep 9 hours or more have ¼ rate of breast cancer of women who sleep 7 hours of less.

2005 Knight JA found positive correlation between duration of exercise and amount of melatonin produced in overnight urine.

2006 Lower incidence of hormone related cancers (breast and prostate) in blind and visually impaired  people.
Schernhammer reported 2005 that incidence of breast cancer was lower in women with higher concentration of melatonin in their first morning urine.

2005 Blask, Brainard et al took blood from women volunteers under three conditions during the day, at night and after light exposure. They then supplied rats with breast cancer tumours on their backs, with blood from these three conditions. They found that the blood with melatonin (night blood) showed no or very slow growth. The two blood types with no melatonin the tumours grew rapidly.

2005 Blask “Evidence is emerging that disrution of one’s circadian clock is associated with cancer in humans, and that interference with internal time keeping can tip the balance in favour of tumour developent”

From press release
Richard Stevens PhD said “If the link between light exposure and cancer risk can be confirmed it could have immediate impact on the production  and use of artificial lighting in this country. This might include lighting with a wavelength and intensity that does not disrupt melatonin levels and internal time keeping.”

“Day workers who spend their time indoors would benefit from lighting that better mimics sunlight,” added Stevens. “Companies that employ shift workers could introduce lighting that allows the workers to see without disrupting their circadian and melatonin rhythms.”

2006 Kuto T et al. found a very significant increase in risk of prostate cancer in men who worked rotating shifts compared to non-rotating shifts. (14,000 subjects, 31 of whom developed prostate cancer)

2006 Lie J A et al. found a 2.2 fold increase in the risk of developing breast cancer in women who worked night shifts for 30 years or more compared to nurses who had not worked nights.

2006 Buja A et al. found a significant increased risk for melanoma nad breast cancer in female flight attendants.

2007 The International Agency for Research on Cancer concluded that shift work is “probably carcinogenic to humans,” based in large part on the growing association between night-shift work and increased incidence of breast cancer (Stevens, 2011; Straif, 2007).

2009 The danish government began paying out compensation to night shift workers who had developed cancer.

No studies so far have tested “time in darkness” as a cancer therapy, although Jim Phelps is using darkness to treat rapid cycling bipolar disorder.

Animal Studies

The following papers can be found by searching ‘continuous light animals’ on Pubmed database. Many thanks to Richard Hansler from LowBlueLights for beginning this search.

1969 Singh wrote 6 papers that showed polycystic ovarian disease in rats resulted from continuous light exposure. Polycystic ovarian cancer can lead to cancer.

1975 Kuralasov found that when transplanting rat mammary carcinoma to a culture dish  in the dark the cultures ‘took’ a smaller percentage of the time and growth was delayed.


1982 Cancer Letters by Kothare LS, Shah PN and Mhatre MC-  Incidence of adenocarcinoma induced in rats under continuous light was 95% compared to 60% raised under a more normal LD of 10:14. Subsequent experiments found removing pineal resulted in the same effect on tumour growth as constant light. Further found supplementing with Melatonin reduced cancer rate but not as effectively as normal light conditions.

1983 Stanberry LR et al found 18 hour nights increased time for tumours to start and decreased rate of tumour growth in hamsters. Concluded quantity, time and duration of Melatonin presentation all had an important effect on tumour growth.


1994 Journal of Biological Rhythms- Nelson RJ and Blom JM Mice with induced cancer, when kept in 16:8 LD near 90% developed cancer. Conversely none of the mice developed cancer when kept under 8:16 LD. A later experiment showed that both male and female deer mice housed in short days had enhanced immune function compared to long day deer mice.

1997 Li JC shifting hours of LD (every three days) from 14:10 to 10:14 resulted in faster growing carcinoma tumours among other negative effects on health compared to mice kept in constant 14:10 LD. I think he meant to type 10:14.

1997 Dauchy et al. Very low light 0.2 lux can increase tumour growth in rats.

2000 Anderson et al found increased incidence of carinogen induced tumours in rats housed in continuous light compared to those raised in LD 8:16.

2001 Beniashvili DS et al found that the cancer rate of offspring of female rats who had been given a carcinogen during pregancy was high for those raised in continuous light, medium for those in  12:12 and low for those raised in continual darkness.

2003 Blask DF et al. Growth rate of human breast cancer grafts increased markedly under continuous light compared to light dark routine.

2004 Moore CB, Siopes TD Chicken hens’ ovarian tumours grew rapidly in LD16:8 but shrank in LD 8:16.

2004 Ferriera AC et al found melatonin output from pineal glands in rats with carcinosarcoma increased 3 times compared to controls 14 days after tumour implantation.

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