Exposure to light at night while sleeping does not increase a woman’s risk of developing breast cancer, a major prospective study funded by charity Breast Cancer Now suggests.

Since the 1970s, exposure to light at night-time has been proposed as a risk factor for breast cancer, possibly due to the disruption of the body’s internal clock—the circadian rhythm—with suggestions that increases in light at night due to industrialization could, in part, explain the rise in breast cancer incidence.

The circadian rhythm is affected by external cues, such as light and temperature, with changes bringing about the secretion of hormones that help regulate the sleep-wake cycle, as well as appetite and thirst.

During a normal sleep-wake cycle, a reduction in light levels results in secretion of melatonin—the ‘sleep hormone’. Previous studies have proposed that disruption to the circadian rhythm caused by exposure to light at night-time may increase breast cancer risk via a reduction in melatonin, and consequently, a rise in circulating estrogen levels.

However, in a comprehensive new analysis, scientists at The Institute of Cancer Research, London, analyzed data from 105,866 UK women from the Breast Cancer Now Generations Study and found no association between exposure to light at night and breast cancer risk.

113,000 participants followed from 2003 onward

The Breast Cancer Now Generations Study is a landmark prospective study of the causes of breast cancer that is following over 113,000 UK women for 40 years.

When participants entered the study, from 2003 onwards, their exposure to light at night and sleeping patterns were established through questionnaires, classifying light levels in their bedroom as low (too dark to see your hand, or you wear a mask), medium (light enough to see your hand in front of you, but not to see across the room), or high (light enough to see across the room, but not read, or light enough to read).

The study followed the participants for an average of 6 years, collecting further data every 3 years, allowing researchers to control for a wide range of potential confounding factors.

Data were gathered on breast cancer risk factors, such as obesity, physical activity, alcohol consumption, family history of breast cancer, age at first period and menopause, number of children and the women’s age at their births, and duration of breastfeeding.

The new analysis was led by Professor Anthony Swerdlow and Dr. Louise Johns at the ICR, and is published today in the British Journal of Cancer.

Researchers at The Breast Cancer Now Toby Robins Research Centre lead important study programs to understand the genetic and environmental causes of breast cancer.

Light exposure not associated with increased risk

The researchers observed that 1,775 out of the 105,866 women went on to develop breast cancer, but found that exposure to light at night was not associated—as previous studies have suggested it might be—with an increased risk of the disease.

This is only the second cohort study worldwide to investigate levels of indoor light exposure at night, and breast cancer risk. Unlike a number of previous studies which have used satellite imaging of street lights to gauge outdoor night-time light levels, this study measured self-reported light exposure, taking into account the light levels actually experienced in the women’s bedrooms.

They found that those who experienced the highest light levels at night were no more likely to develop breast cancer than those who reported low-level light exposure, and that exposure to light at night had no effect on risks of receptor subtypes of the disease. There was also no effect of light at night when premenopausal and postmenopausal breast cancer risks were considered separately.

Adjusting for various sleeping patterns—non-peak sleep (going to sleep after 2 a.m., or waking before 1 a.m.), sleep duration, or history of night shift work in the 10 years prior to recruitment—did not appear affect the light at night results.

Further research will be needed

Study leader Anthony Swerdlow, professor of epidemiology at the ICR, said:

“The theory that exposure to light at night might affect a woman’s risk of breast cancer dates back 30 years. It remains an intriguing but unproven hypothesis. Our study suggests that light levels at night do not materialy increase a woman’s risk of breast cancer, but further research will be needed before a definitive conclusion can be reached.”

Baroness Delyth Morgan, Chief Executive at Breast Cancer Now, which funded the study, said:

“By more accurately assessing the light levels women actually experience in their homes, this study provides the strongest evidence to date that exposure to light at night does not increase breast cancer risk. As the Breast Cancer Now Generations Study continues to uncover more about the causes of the disease, we need to turn this into practical guidance to help women reduce their risk, and into ways to determine how and when breast cancer is likely to develop.”

With more women being diagnosed with breast cancer than ever before, the Breast Cancer Now Generations Study—which launched in 2003 and is based at the ICR—continues to investigate the genetic, lifestyle, and environmental factors that may influence a woman’s risk of developing breast cancer.