Don’t Sleep with the Lights On!

It was over 23 years ago when Dr Richard Stevens, Professor of Cancer Epidemiology at the University of Connecticut, first suggested that there might be a link between electrical light and breast cancer. At that time, cancer researchers just laughed and dismissed the idea. However, recent studies have borne out the theory that women who have lights on in the bedroom are at greater risk for breast cancer than those who sleep in total darkness. The latest study, which comes from Israel, contained information from 1,679 women and is published in the journal Chronobiology International.

Information was obtained at interviews on exposure to light-at-night in the bedroom, including sleeping with the TV switched on; the women were asked to rate their night-time bedroom light levels with a simple scoring system: 1 represented complete darkness, 2 was low light, 3 was average light, and 4 was very strong light i.e. all the lights were on.

After controlling for education, ethnicity, fertility, and alcohol consumption, light-at-night was significantly associated with an increased risk for breast cancer by 22%.

The investigators claim that this is the first study to have identified an unequivocal positive association between bedroom-light intensity and the risk for breast cancer. However, some experts dispute the word "unequivocal", as the data were obtained by recall, allowing for considerable bias. Also, there was a large difference in the response rates – only 52% of controls (i.e. non-breast cancer subjects) as compared with 86% of cancer cases agreed to answer the questions on light in the bedroom.

The lights-at-night theory suggests that there is disruption of the circadian rhythm, similar to that which occurs in shift workers. Shift work is already accepted by some authorities as a carcinogenic risk to humans – in Denmark compensation has been paid to women who developed breast cancer after long spells of night shift, while working as nurses or flight attendants.  Even reduced or dim light in the 8 hours preceding bedtime can suppress the onset and duration of synthesis of melatonin, according to a study reported in the Journal of Clinical Endocrinology & Metabolism.  And, exposure to room light during the usual hours of sleep suppressed melatonin by more than 50%.  Melatonin has been shown to exert an inhibitory effect on breast cancer cell proliferation, mediated through an effect on estrogen receptors, which may help explain the mechanism of the light-at-night and breast cancer in women. 

A lot of this is a bit speculative, but until we know better, it’s enough for me to advise women to heed the warning – don’t make a habit of going to asleep with the TV or the lights on.