Evening light is far worse for your body's built-in clock than you thought

For hundreds of thousands of years, humans lived under one source of light: the sun. It rose in the morning. It set at night. And evolution programmed our bodies according to that ancient, never-changing schedule.

Then we invented electric lighting.  Now instead of spending our days under the bright sun, most of us spend them in rooms that are relatively dim.

And instead of spending our nights in darkness, we spend them in rooms that are relatively bright.

"The way we evolved, we were only in either very bright or very dim light, " said Sean Cain, from Monash University's Turner Institute for Brain and Mental Health. "But now, we are living in this 'twilight zone' almost all the time."

Cain is a circadian biologist who investigates the impact light has on our bodies' built-in clocks, and his latest study illuminates how striking that impact really is. He and a team of researchers which included Andrew Phillips, Parisa Vidafar and their colleagues recruited 55 study participants: a mix of men and women aged 18-30. On several evenings spaced out over about six weeks, these guinea pigs were exposed to varying levels of light for five hours at a time: ranging from 10 lux (an intensity that'd make you complain It's so dark right now I can barely see a thing) up to 2000 lux (Does it have to be so darn bright in here?).

During the differing light levels, researchers measured how each intensity affected the release of the sleep hormone melatonin. "Melatonin is referred to as a hormone of darkness," Cain explained. "It tells your body, not that you're evil, but that it's nighttime."When light is detected by photoreceptors in the eyes, your body interprets that as a sign the sun must still be high in the sky — melatonin is suppressed, and you're kept awake. No light, or falling light detected — melatonin is released, and you're lulled towards sleep.

While this link between light and melatonin is pretty well-established, Cain and his colleagues were surprised to discover that light exposure had a far greater impact than they anticipated.

The results of their study are published in the journal PNAS.

"On average, people are about three to four times more sensitive to just broad spectrum white light than we previously thought," Cain told Coach. As a group, the participants suppressed half their melatonin when exposed to less than 30 lux, about the brightness of a typical bedside lamp. "That essentially delays that signal for night by about 77 minutes," Cain said.

Exposure to 50 lux — about as a bright as a standard living room — delayed the signal for the onset of night by 109 minutes, on average.

melatonin sleep light night

In much the same way that a simple '90s laptop built for word processing and Solitaire would struggle with Facebook and YouTube, your body's ancient programming struggles in these strange modern light patterns.

"Our bodies are trying to shut down [at night] and we're under all these lights that are saying, 'No, you're wrong about what time of day it is. It's actually still daytime. It's not time to go to bed yet,'" he said. "It's really confusing."

Some study participants' bodies were far more confused by evening light than others. "Some people were reacting to almost no light," Cain said. "They were suppressing 50 percent of their melatonin to only about 6 lux, which is dimmer than almost anyone has it in their homes." Others didn't suppress that much melatonin until up to around 400 lux. "Which is very, very bright [for] indoor light ... brighter than anyone's home," said Cain. "It's more like what you get when you go outside maybe on an overcast day."

Overdosing on light at night might do far worse than leaving you feeling a l'il tired the next day because you couldn't get to sleep on time. Cain and his colleagues believe upsetting your body clock — aka your circadian rhythms — has consequences for long-term health.

Earlier this week, fellow Monash scientist Professor Paul Zimmet proposed "circadian syndrome" may even be behind the global epidemics of obesity, heart disease and diabetes.

"Our use of artificial light is potentially a real health hazard... it's not how we evolved," Cain said. "Our bodies were made to be aligned with a very, very predictable, non-controllable 24-hour cycle, and we're mucking it up. It is probably a foundational problem for a lot of disease vulnerability."

This article was first published by Sam Downing on the coach.nine.com.au blog


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