Electric Lights Disrupts Human Circadian Rhythm

MedicalResearch.com Interview with:
Richard G. Stevens, Ph.D.,
Professor, Cancer Epidemiologist
UConn Health

Medical Research: What is the background for this study? What are the main findings?

Dr. Stevens: Since first introducing the concept of a possible connection between exposure to light at night and breast cancer in the mid-80s, we’ve seen growing evidence of how artificial light can suppress the circadian hormone melatonin and bring about physiological changes.

The extent of this “circadian disruption” varies by the type of light and the time of day. Humans evolved with a body clock that followed the solar clock. Nature intended us to be awake in daylight and at rest in the dark of night. Therefore, the intense, short-wavelength light of the sun in the morning triggers us to become awake and alert, just as the absence of sunlight in the evening allows our body to produce melatonin. Even with the use of fire to provide light in the evening, the circadian impact was relatively minimal because of firelight’s place on the red end of the visible spectrum.

Humans survived under this simple formula for many thousands of years. Then electric light started to take an increasingly strong foothold in everyday life. Today we are typically surrounded at all hours of the day and night by artificial light – in many cases it’s not bright enough during the day to match the sun, and it’s too bright at night to be conducive to the natural sleep/wake cycle. Think computer screens, tablets, smart phones, e-readers, etc. These devices emit enough short-wavelength, or blue, light to disrupt our body clocks in the evening. So do fluorescent and LED lights.

Our paper – I worked with Dr. Yong Zhu from Yale on this – represents a new analysis and synthesis of what we know up to now on the effect of lighting on our health. We don’t know for certain, but there’s growing evidence that the long-term implications of this may have ties to breast cancer, obesity, diabetes, and depression, and possibly other cancers.

Exposure to electric light  started about 130 years ago,  which is a tiny period of time in evolutionary terms. In other words, not long enough to undo human evolution.

Medical Research: What should clinicians and patients take away from your report?

Dr. Stevens: An understanding of the importance of maintaining the sleep/wake cycles, and the effect even typical lighting in the modern world can have on it, could go a long way in staving off the potentially harmful long-term impact. Our smart phones and LED bulbs can’t disrupt our body clocks if we don’t let them. We can opt for reading a book under an incandescent light before bed, rather than on an e-reader with a blue backlight. And because we’re gaining more of an understanding about this, we’re starting to see our technology come equipped with the option of adjusting the type and intensity of light it emits.

Medical Research: What recommendations do you have for future research as a result of this study?

Dr. Stevens: Although studying people assigned to “dark” and “light” groups may tell us a great deal, we can’t ethically do that. What we can do is continue to study people who already have lifestyles that would be of interest to the science. We can survey third-shift workers and compare their  health experience to day workers, for example. Or we can follow children who grow up in a rural area who don’t use smart phones and compare them to city dwellers who do. We certainly can do studies with animal models. But perhaps most important would be an understanding of the possibility of long-term health effects associated with artificial light. If we’re aware of it, perhaps we can take steps today to reduce our chances of  illness tomorrow.


G. Stevens, Y. Zhu. Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem? Philosophical Transactions of the Royal Society B: Biological Sciences, 2015; 370 (1667): 20140120 DOI: 10.1098/rstb.2014.0120

[wysija_form id=”2″]


MedicalResearch.com Interview with: Richard G. Stevens, Ph.D. (2015). Electric Lights Disrupts Human Circadian Rhythm