Artificial light, sleep and the battle to keep our circadian rhythms in tune

In his New York Times bestselling book Dreamland – Adventures in the Strange Science of Sleep, journalist David Randall devotes a lot of words to Thomas Edison, the inventor of the lightbulb – “… a product that was once revolutionary and now costs less than $2”. Artificial light changed everything for human behaviour and sleep, he writes. Night became an artificial day. 

Monash University Associate Professor Sean Cain, within this broad idea, is looking at circadian rhythms and melatonin, which regulates sleep. He's also looking closely at evening light; recently, he was the senior author on a paper in the US science journal PNAS. 

“Every organism, even a single-celled organism, shows 24-hour rhythms. The rotation of our planet is stamped into the physiology of every organism on it."

It found we are “highly sensitive” to artificial light in the evening, with a remarkable 50 per cent suppression of melatonin at light levels the same or lower than house lights and device screens.

The researchers also found massive differences in sensitivity to this light across individuals, which they think may explain sleep disorders and their impact on health.

The problem is that light at night is fundamentally unnatural and disturbs melatonin (a hormone) and those inbuilt circadian rhythms. Our bodies want to have a daytime and night time mode (like activity and repair), and light at night confuses these processes. 

Associate Professor Cain, from Monash’s School of Psychological Sciences, says we're made of clocks, and they're intrinsic to our being. 

“What I find really interesting about circadian rhythms is, they are a fundamental system,” he says.  

“Once you start disrupting the rhythms, you make the foundations of your body unhealthy."

“Every organism, even a single-celled organism, shows 24-hour rhythms. The rotation of our planet is stamped into the physiology of every organism on it. 

“Virtually every tissue in the body can generate its own 24-hour rhythm. We have a central clock in the brain, but there are also rhythmic tissues in the brain, the pancreas, heart cells, skin cells. They are all rhythmic. We're made of clocks. 

“Once you start disrupting the rhythms, you make the foundations of your body unhealthy. The manifestations of these unhealthy rhythms may depend on an individual’s vulnerabilities. This may be weight gain in some, poor mood in others.”

House lights should be dim in the evening and at night, he says, and in his own home he has a smart-light system whereby his lights turn dim and turn orange at dusk. The contention here is that by brightening things artificially in the evening and at night, the melatonin that sends us to sleep is restricted. “We need the melatonin rising in the evening as a signal for night and a signal for sleep,” he says.

Associate Professor Cain is from Canada. He's been studying circadian rhythms and their impact on cognition since the late 1990s. He came to Monash in 2012 from Harvard Medical School in the US. 

Dimming it down

The PNAS study examined 55 men and women over six to seven weeks of light testing, and found even dim reading lights delayed the release of melatonin by 77 minutes on average. 

Modern LED lights are blue-enriched, which is especially problematic. 

“Our clocks are especially sensitive to blue light, so even if our home LEDs and smartphone lights are quite dim, they would still be giving a signal to our clock that it's daytime,” he says. “They might look white, but there's a blue spike.”

He says device screens and OLED/LED TVs in the evening and at night could be a massive problem for some people. Even a quick check of a device before bed – or in bed – could potentially be disruptive, he says. It’s to do with the difference between visual photoreceptors and circadian photo-reception.

“It could matter if you look at a phone for only a little bit of time in bed, because circadian photoreceptors turn on and off very slowly. 

“In order to have a visual experience, your visual photoreceptors need to turn on and off. They do this very quickly. But circadian photoreceptors evolved to tell whether it's day or night. 

“They're slow to turn on and slow to turn off. In tests, we found that even after five minutes of blue light, human eye pupils are still behaving as though they're in light when they're actually in the dark. 

“People are exposing themselves to very bright light all the way up until bedtime, and what we show in this paper is that the average person suppresses about half their melatonin with only about 30 lux of light – which is reading-lamp light.”

Associate Professor Cain knows he's fighting a strong collective bias in society – deeply rooted from ancient times – that light is good and dark is bad. 

“We feel safer in the light. It's physiological, it's built into us. But if there was a nocturnal human, the mythology would be flipped – light would be bad. Nocturnal animals are afraid of light. If they could express themselves like we can, they would say light is bad, and darkness is safe and nice and beautiful.”

A seemingly small thing, like picking up the phone in bed as you prepare for sleep, is subconscious, he says.

“We're unconsciously using the phone as a light delivery device. We want the effect of the light. When you pick up the phone in bed, it's not about what you do on the phone, it's about the light. 

“We seek light and we are rewarded by light, and now that we have complete control of our light we don’t have to wait for the sun to come up; we can flip a switch and ‘bam!’, we have light and everything that comes with it – enhanced mood, enhanced alertness, more feelings of safety. But there's a cost.”