When Night Stops Falling

In Isaac Asimov's seminal and award-winning short story Nightfall, a planet with six suns descends into anarchy and madness when they all align and are eclipsed at once, plunging the planet into darkness. An epidemiologist at the University of Connecticut's School of Medicine believes we on earth may be in trouble for just the opposite reason.

Professor Richard G. Stevens upholds that artificial sky glow – reflected light scatter in the atmosphere from the electric lighting below – is disrupting our circadian physiology.

Our circadian rhythms encompass all of the regular mental, physical and behavioral changes that occur within us in that 24-hour cycle we call a “day.” Humans, like most other lifeforms on the planet, possess an”endogenous circadian rhythmicity,” a built-in cycle for sleep and wake patterns, hunger, activity, hormone production, body temperature and many other physiologicalprocesses. In fact, studies show that the circadian cycle controls from 10 to 15 percent of our genes. These rhythms take their cues from the onset of light and darkness.

So, what happens when it never gets dark? Stevens and others believe that we are already feeling that impact, in the form of poor sleep, obesity, diabetes, certain cancers and mood disorders.

“This circadian physiology has developed over billions of years. Humans have been living with electricity only since the late 19th century, and with widespread access in industrialized countries only since the 20th century. While that sounds like a long time, it’s a tiny drop in the evolutionary bucket. We are only beginning to understand the health consequences artificial light has on our circadian physiology,” he writes in the online publication The Conversation.

A new “World Atlas of Night Sky Brightness,” just published by Science Advances, underscores just how grand is the depth and breadth of the artificial sky glow. The Milky Way is no longer visible to one-third of humanity, with 60 percent of Europeans and 80 percent of North Americans can no longer see the any portion of the rest of their galaxy at night. (The worst place for sky glow? In Singapore, the light pollution is so bad that the entire population lives under skies so bright that the eye cannot fully dark-adapt to night vision.)

Stevens acknowledges that the sky glow reported in the atlas is, by itself, probably below the threshold for directly affecting our circadian rhythms, as measured by suppression of the circadian hormone melatonin. But he points out that the sky glow the atlas measures is the atmospheric reflection of electric lights in the immediate human environment. Those local light sources are in many, if not most, cases sufficient to cause circadian disruption. These include the lighting inside homes and commercial buildings as well as some forms of street lighting.