PERFECT TIMING
Our circadian clock affects when and how well we all sleep. Yet we also have ‘clocks’ throughout our brains and bodies that need to be working in sync to create optimal health.
We all have biological ‘clocks’ in our brains and bodies that need to be working in sync to create optimal health.
For paramedic Lisa, starting shiftwork nine years ago led to weight gain, reduced physical activity and mood changes. “You’re awake at times you’re not meant to be awake,” says the mother-of-two in her mid-thirties. “My body doesn’t know whether it’s day or night.”
You’ve probably heard of circadian rhythms, the body’s daily cycles controlled by a clock in the brain. But did you know that organs and cells each have clocks that are exquisitely tuned to natural rhythms? New research is unveiling the complex mechanisms controlling internal clocks, and their profound implications for health.
Circadian clocks are actual physical clocks within the body, says Sean Cain, associate professor of psychology at Monash University. The master clock, which creates 24-hour rhythms, is in the hypothalamus at the base of the brain, explains Cain, who is also president of the Australasian Chronobiology Society. “But there’s also clock tissues throughout the brain and body and they have a relative timing and synchrony with each other that creates optimal health,” he says.
“However, these days we tend to do ourselves a disservice with light environments and behaviours that put our clocks out of sync and create a greater likelihood of poor health.”
ROLE OF CIRCADIAN CLOCKS
Cain says research is highlighting how circadian rhythms influence almost every aspect of health. His Australasian Chronobiology Society colleague, Dr Andrew Phillips, explains they affect us in three main ways.
The first is sleep. “The central circadian clock controls when we feel sleepy and when we feel alert,” he says. “If sleep is misaligned from where the body is optimally prepared for it, then sleep quality as well as quantity are reduced.”
The second is metabolism. “There’s an expanding field looking at how disruption of circadian rhythms is associated with impaired metabolism and disease states, like diabetes and obesity, in groups that are chronically exposed to circadian disruption,” says Phillips. “Shiftworkers, for example, tend to have many more health complaints, especially metabolic health complaints. It’s also recently been found that people who have different routines from one day to the next are at much higher risk of poor cardiometabolic outcomes. There’s this daily rhythm of anticipating when food should be available and if that’s disrupted, it seems to impact many aspects of our body’s clockwork.”
This happened to Lisa when she started working shifts. “I had a big issue changing my physical habits – not wanting to go out and exercise and not eating properly,” she says. “We have programmes that give you nutritional advice. You know you’re meant to eat carrot sticks, but at 3am, when you’ve been up for a lot of hours and you haven’t slept the day before, you’re snacking on whatever you find.
“Over time you learn to cope a little bit better, but even now I still get quite nauseous on a night shift, which can impact my eating habits ... You want something to get you through the night shift – a sugar hit or a caffeine hit,” she says.
Lisa also found shiftwork affected her mood, a third major area tied to circadian clocks. Irregular schedules and lack of sleep sometimes left her tired and irritable. “The kids knew there’d be days when Mum was cranky because she’d worked last night,” she says. “Being angry affects my mental health because I know I’m not being the best version of me.”
Associate Professor Cain explains that the circadian system is fundamental to our moods. This is partly related to how well we sleep. “We know that with really poor sleep, there’s more impairment of the prefrontal cortex, which results in us being less able to inhibit negative thoughts. When we’re tired, we tend to be more negative and in a poorer mood,” he points out.
But Cain says there’s also a direct relationship between the circadian system and mood. “It’s known that in depression, there tends to be disrupted rhythms. Now we’re realising our exposure to light patterns is important for maintaining good mood.”
When it comes to regulation of circadian rhythms, light exposure is by far the most important factor, Dr Phillips says. He, Cain and their research team have shown humans are, on average, highly sensitive to evening light. Their research,
published last year in Proceedings of the National Academy of Sciences of the United States of America, studied how exposure to different light levels affected melatonin (a hormone that helps regulate sleep/wake cycles) in 55 people over six to seven weeks.
Darkness allows the body to produce more melatonin, which signals it to prepare for sleep. The study found a 50 per cent suppression of melatonin occurred at less than 30 lux, which is comparable to (or lower than) typical indoor lighting used at night, as well as light produced by electronic devices.
Interestingly, they also found a more than 50-fold difference in evening light sensitivity between individuals. This finding may help to explain why some people are more vulnerable than others to circadian disruption and its effect on health.
LIGHTING THE WAY
Light’s role is so crucial that even tissues deep within the body can sense it, new research suggests. A team from the University of California, Irvine, found a way to switch off all the clocks in mouse models, then restart them in specific cells.
Lead researcher Professor Paolo Sassone-Corsi says their findings, published last year in the journal Cell, showed that the liver and skin knew what time it was, even though the central clock had been shut down. “We’re talking about a mouse which can’t see light as far as the clock is concerned, so I found this surprising,” says Sassone-Corsi, who is also the director of the university’s Center for Epigenetics and Metabolism.
In the documentary Nightbirds: The Magic of Night, Sassone-Corsi explains that our genes are intricately timed with day/night cycles, like a conductor directing the timing of instruments in a symphony.
During the night, for example, cells are supposed to repair stresses accumulated during the day, especially in the skin where UV exposure can cause damage.
However, the advent of electricity changed things. “For our biology it was almost like a stick of dynamite going off,” he says in the documentary. “The difference between night and day has become less discernible ... We have light everywhere, all the time, shining strong, high-energy photons in our face at the wrong time when we’re supposed to be in complete darkness.
A large number of our physical problems are related to the misalignment of our internal clocks [which] is caused by lack of definition between day and night.”
Furthermore, he explains that while clocks can work independently of each other and the primary clock in the brain, they usually communicate with each other. This communication is the basis of homeostasis – the process living things use to maintain the stable conditions needed for survival. “Good homeostasis means better health, better ageing,” he says.
Also influencing circadian rhythm is when we eat and exercise. “If you have a cheeseburger or go for a run at midnight, those cycles are disrupted,” says Sassone-Corsi. “You activate inflammation, so you’re creating a stress.”
But curbing the urge to hit the fridge at night could be difficult, with a new study indicating that the brain’s clock and its pleasure centre are linked. Published in January in Current Biology, the research (which used mice) also found that highcalorie foods (which bring pleasure) disrupt normal eating schedules, resulting in overconsumption and eventually, obesity.
One of the researchers, associate professor Ali Güler from the University of Virginia, explains what this means for humans: “When people consume foods with fat and sugar, they feel good partly because their dopamine system – which responds to rewarding events – gets activated. We believe this release of dopamine also reduces the effectiveness of the neurons in the brain that make up the biological clock.
“The biological clock is important for health as it times all metabolic functions. When one consumes energy-rich foods outside of the normal mealtime (that is, snack), it throws this intricately timed system out of whack.
“When you open the fridge in the middle of the night, a time the body does not normally need more energy, the biological clock might be saying it is not time to eat, but the reward centre would override that signal by interfering with the neurons controlling the circadian rhythms. In modern times, in front of a fridge, we have access to ‘the spoils of a hunt’ at any time of the day. It is no surprise that we consume highly caloric, tasty foods all the time.”
“MANY PHYSICAL PROBLEMS ARE RELATED TO MISALIGNMENT OF INTERNAL CLOCKS.” PROF PAOLO SASSONE-CORSI
EFFECT OF MEDICATIONS
Medications can also impact your clocks. Associate Professor Cain explains that research they conducted in 2018 showed that selective serotonin reuptake inhibitors (SSRIs), used to treat depression, can greatly increase the clocks’ sensitivity to light. The positive side is that the mood-boosting effects of daylight may be enhanced. Conversely, people on SSRIs are more susceptible to the negative effects of light exposure at night, like melatonin suppression. He says these medications might contribute to a “jet lag-like” feeling, especially in patients with very irregular light exposure patterns, and hopes doctors will advise patients about this newly understood side effect, so they know to be more careful about their light at night. While this avenue of research is in its infancy, it’s likely that other medications could also impact circadian clocks.
Despite all the negatives associated with irregular routines, paramedic Lisa emphasises that shiftwork has pay-offs. “The mental and physical benefit is that we get extra days off during the week,” she says. “I do have days where I can go paddle boarding and to the beach.
“There is a recovery phase after night shifts in restoring that rhythm, but there’s also extra days to enjoy life and maintain a positive outlook and healthy activities.”