The circadian rhythm of life: ‘If we disrupt the light cycle, then you’ll die earlier’

“It’s hubris,” says Monash’s Associate Professor Sean Cain, a global expert on light and the human mind and body. “Hubris”, as in, an arrogance that’s blind to risk.

“Virtually all cell types in our bodies have 24-hour rhythms, so the cycles of light and darkness are stamped into all of us. Yet we think we're going to get away with behaving in this way. Everything is disrupted. We need to go back as much as possible to how things used to be.”

Associate Professor Cain, from the School of Psychological Sciences, has been researching light and human circadian rhythms for many years. He’s a “circadian biologist”, interested specifically in how different light – healthy and unhealthy – affects us physically and mentally.

Unhealthy light – or at least too much of it – is artificial light at night. In the natural world of dawn and dusk, it’s not there. This is the hubris – that we think we’ll be fine if we mess up the natural, ancient circadian rhythms.

“We evolved over millions of years with no control of the light,” Associate Professor Cain says. “Bright days, dark nights. Now, in the past 150 years, it’s a very, very different environment for our bodies, and it’s difficult to cope with that. We didn’t evolve to control our light environments, and we tend to make bad decisions about it.”

He likens the problem to the difference between good and bad foods. “After years of having massively overprocessed foods, people are appreciating more natural foods, more unprocessed, more organic. We should be doing that around our light cycles, too.”

Potential for huge societal impact

Associate Professor Cain has now co-led a “massive” international study, published in Nature Mental Health, showing that brighter days and darker nights are fundamental for mental health. It’s the largest study on individual light exposure conducted to date.

“It’s been a long time coming,” he says, “and I consider this our group’s greatest-impact work. I think it'll have a potentially huge societal impact.”

The paper explains the human body’s circadian clock, which is fuelled and recharged by natural light. This central clock sits in the suprachiasmatic nuclei of the hypothalamus in the brain, and regulates the timing of basic cellular functions, physiology, cognition, and behaviour.

Rhythms within it are regulated by light exposure patterns, yet “humans in modern, industrialised societies challenge this biology, spending [about] 90% of the day indoors under electric lighting, which is dim during the day and bright at night compared with natural light/dark cycles”, the paper says.

“Deviations from our natural light/dark cycle lead to disrupted circadian rhythms throughout the brain and body, and therefore could contribute to adverse psychiatric outcomes.”

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

The researchers examined the data of 86,772 people, looking at light, sleep, physical activity and mental health. They found greater night-time light exposure was associated with increased risk for major depressive disorder, generalised anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm.

Conversely more natural light during the day meant reduced risk for major depressive disorder, PTSD, psychosis, and self-harm behaviour.

The extraordinary findings have led to (so far) more than 130 news articles internationally, and also a place in one of the world’s leading health podcasts, the Huberman Lab. The paper was on the cover of Nature Mental Health when it was published.

“There’s a clear and reasonable pathway for why the light at night would cause disruptions of rhythms, and therefore mood disorders,” he says. “But it’s also the case that light does put you in a better mood. So if you’re experiencing mental illness and you’re not feeling well, it’s a self-medication as well.

“People are drawn to light, especially when they’re not feeling well. So unfortunately we’re drawn to light at night. It makes us feel better in the moment, but makes us more ill.”

Evidence confirms artificial light harms

The weight of evidence is now firmly about avoiding too much artificial light – and blue LED light from screens – after dark.

“Light at night is disruptive, and that disruption leads to mental health illnesses and many other kinds of illness, especially things like cardiovascular illness and diabetes,” Associate Professor Cain says. “Pre-clinical studies have shown for years, if we disrupt the light cycle, then you’ll die earlier.”

The best-case scenario for a human in 2023, he says, is the following, based on his personal experience:

“If you’re more of a morning type, you’ll get bright light in the day and likely less light at night. Morning types tend to be quite regular in their timing. And I say early types, but people talk about that as though it’s set in stone.

“I used to have delayed sleep-wake phase disorder, so I used to find it very difficult to get to bed before 2am. And my circadian system is incredibly sensitive to light, so even a little bit of light at night will cause me to stay up late very easily, so I control my own light exposure.

“I very purposefully get lots of light in the day. I have smart lighting in my home, which is very dim at night and goes on and off at the same time every day.

“Now I would be seen as a morning person. By nature, I would tend to be more of an evening type. But when I just get natural light cycles, I’m a morning type and I’m very regular, which is what the typical healthy person would look like.”

And an unhealthy one?

“They would be getting very irregular light, lots of light at night. It causes their rhythms to be disrupted. So from one day to the next they’d be getting up and going to bed at different times. They'd be less in sync with their natural light-dark cycle.

“We know that those types tend to be more vulnerable to pretty much all illnesses, especially mental illness. If you look at people with major depressive disorder, they have more light at night, more irregular schedules, and weaker internal rhythms.”

Light sensitivity and antidepressants

Associate Professor Cain has a A$3.2 million Wellcome Trust grant to look at how light behaviour impacts how well antidepressants work after discovering several years ago that antidepressants can increase the sensitivity of the human circadian system to light.

“When we say ‘looking on the bright side’, I think that’s literal,” he says. “We’ve shown that someone in a current state of depression has a less light-sensitive circadian system. I think that's the origin of this idea of depression being a feeling of darkness, because this unconscious brightness detection system that feeds into the clock and mood-related areas of the brain isn’t seeing light in the same way.”

Read more: Let there be light – but make sure it’s the natural, healthy kind

This newest study focuses on the importance of people’s light exposure patterns on how well medications work.

The theory is that when people seek “healthy light patterns” – bright days, dark nights, regularity – they’ll do better because the medications will boost light without the bad effects.

“Whereas if people have irregular schedules, more light at night that's more disruptive to the clock, then boosting light input will have a negative impact. It’ll cause a sort of perpetual jet lag,” he says.

Indoor and outdoor light aren’t the same

Even if a person thinks they get enough sunlight during the day and not too much artificial light in their homes at night, they’re probably incorrect, he says.

“Visually, you don't see a big difference between being indoors by a window and being outside. But the circadian system sees a big difference.”

“You would think we do get lots of light in the day, and that should be fine. But many human beings spend about 90% of their time indoors. Even though you’re sitting indoors by a window and you're getting some light, it’s nowhere near the same as being outside. “Visually, you don't see a big difference between being indoors by a window and being outside. But the circadian system sees a big difference.”

Read more: The dark side: How too much light is making us sick

When the sun goes down, it’s time to also dim the lights. It all makes sense. How can we expect to do well when we shift the internal body clock so much that it doesn’t know what time it is?

“As for light at night,” Associate Professor Cain says, “there’s a lot of focus on screens and phones, but that’s a bit overblown. The worst offender is house lights. Downlights are horrendous, and so many people are living in light environments at night that are disruptive, and they’re also spending too much time indoors during the day. It’s both sides. They're equally important.”