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Serendipity (coupled with sharp observation) struck twice in Randy J. Nelson’s lab, at Ohio State University. He first began investigating the effect of light on mood disorders when a graduate student in the lab complained about high levels of light during the night, when hospitalized with a Staph infection. This triggered research on the effect of constant light on mood disorders, using male mice as the test subjects. The second ‘light bulb’ moment occurred when the team noticed that their depressed and anhedonic mice were becoming fat and glucose intolerant, even though their diet hadn’t been altered. Connecting this observation with earlier studies that reported shift workers with elevated BMIs, and even cancer, led to research published in PNAS by Nelson and colleague Laura K. Fonken (evaluated here), and explored further in a recent F1000 Report. Nelson talked us through this research at SfN last year https://blog.f1000.com/2010/11/19/all-through-the-night/.

It seems that your friendly streetlight may be a public menace, triggering serious health problems such as obesity, diabetes, and depression. At the center of the issue is melatonin—a pineal gland hormone whose secretion or suppression is mediated by light exposure. Just as light tends to dispel monsters under the bed, it also inhibits melatonin secretion, with scary consequences for those of us who live increasingly illuminated lives.

Fonken and Nelson also report that melatonin may act directly on tumor cells, inhibiting growth. Low levels of melatonin, conversely, may increase signaling of estrogen/progesterone, hormones that promote breast cancer. Melatonin’s effect on oxidative stressors may also delay tumor initiation and progression. Furthermore, circadian confusion induced by light-sensitive phenomena like jet lag has been shown to accelerate tumor development in clock-gene knockout mice (see these evaluations, with a dissent and author rebuttal on the latter topic).

And it’s not just jet-setters and shift workers who are at risk from the effects of light confusion: electric lights have expanded all of our daylight hours by four to seven hours. Even if you do live in a place where it actually gets dark—really dark—at night (and a glimpse of this map will likely verify that you don’t), the type of lighting you encounter indoors could also knock your circadian rhythms out of balance. That’s because retinal ganglion cells reporting to the brain’s circadian circuitry are most responsive to blue wavelengths of the sort common in computer screens, compact fluorescent bulbs, iPads, television sets—in short, all those things we view at night instead of the backs of our eyelids.

The authors suggest a few “relatively low-cost manipulations” that any of us can do to reduce exposure to extraneous light: closing the curtains, turning off hallway lights, and removing televisions and computers from bedrooms. They also suggest keeping a regular schedule, and avoiding sudden shifts in waking hours like those experienced by shift workers. For those who can’t avoid making such shifts, the authors bring up the prospect of wearing goggles to screen out blue light. Good luck with that.