A new study estimates that researchers found astronauts on the International Space Station found that exposure to high levels of outdoor artificial light at night (ALAN) in the blue spectrum was associated with an increased risk for two cancers. The study was published online in Environmental Health Perspectives.

The findings are from an observational study and therefore are not definitive. Investigators evaluated exposure to ALAN among individuals who had never worked at night and who were part of a population-based, case-control study known as MCC-Spain. For the study, the team enrolled 1219 breast cancer patients, 1385 women who did not have cancer to serve as control participants, 623 prostate cancer patients, and 879 men who did not have cancer to serve as control persons from 11 regions in Spain from 2008 to 2013.

The investigators determined exposure to indoor ALAN using questionnaires. Outdoor ALAN exposure was analyzed using images from the International Space Station that were available for Barcelona and Madrid for the period 2012-2013. "Exposure to ALAN is ubiquitous and whether the spread of exposure to ALAN may increase cancer risk and how this could be prevented are public health issues," they add.

In the study, estimates of exposure to indoor ALAN in the full light spectrum, using participant questionnaires and face-to-face interviews, demonstrated an association with an increased risk for prostate cancer but not with an increased breast cancer risk, the investigators say. Prostate cancer was positively associated with the highest level of indoor illumination in the full light spectrum during bedtime compared with no indoor illumination (OR = 1.80).

Conversely, there was no evidence of an association between indoor ALAN in the full light spectrum and increased risk for breast cancer (OR = 0.94). This is also the first time that the spectrum of emissions from nighttime lighting has been measured, the researchers note. Prior studies have relied on data from "color-blind" satellite images that detected light but did not measure the spectrum of emissions from nighttime lighting.

The study authors point out that light emitted by portable electronic devices with self-luminous displays and energy-efficient lighting may contribute to indoor ALAN exposures. The use of such devices is increasing and has a significant effect on decreasing melatonin production if they are used before bedtime.

In the Previous study, researchers demonstrated an overall higher risk of breast and prostate cancer in night shift workers who were exposed to high levels of indoor ALAN from a study population of 4106 participants. For the current analysis, they enrolled persons who had never worked the night shift. Clinicians should consider evening and nighttime lighting in patients' homes when assessing health, advised Stevens.

Lightbulbs that are relatively dimmer and that emit light of longer wavelength are best, he said. He noted that in his bathroom, he uses a nighttime plug-in with a dim red light. In the absence of electric light, the body "gets ready for bed" by increasing the propensity to sleep after sunset. Melatonin levels rise gradually, the metabolism slows, and levels of leptin increase, dulling hunger. In today's fast-paced world, however, many are exposed to too much bright light and light in the blue spectrum at night.