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Keeping time: how our circadian rhythms drive us

Late nights and jet-lag see us fighting our body clocks, but can we ever win? fmgbain/Flickr

Do we control our body clocks or do those clocks, ticking imperceptibly, control us? It’s the kind of question that keeps sleep scientists awake at night.

Rhythms are a good place to start. They are a part of life on earth. The cycle of the seasons, the cycle of day and night and the cycle of the tides are all examples of the constant and reliable rhythms that impact our lives.

Flowers and leaves open to the sun, early birds catch the worms, we rest at night time.

In humans and other mammals, the “drummer” that keeps the rhythm going is located in the brain or, more specifically, a group of cells at the base of the hypothalamus called the suprachiasmatic nucleus.

This natural pacemaker taps out your body’s “circadian rhythms” including the 24 hour cycles that regulate core body temperature, the production of hormones like melatonin, heart rate, blood pressure and digestive functions.

The most obvious of these circadian rhythms is our sleep/wake cycle.

If you put a human into an environment without any light/dark signals or any other time cues, they will fall asleep approximately every 24 hours, their body temperature will rise and fall approximately every 24 hours, their pineal glands will excrete melatonin approximately every 24 hours.

You can essentially tell what time it is on their body clock by watching their behaviour. So is it just coincidence that our rhythms match so well to the daily light/dark cycle?

Of course, the answer is no. We have evolved to be active during the daylight hours and inactive during the dark hours. From an evolutionary perspective, maintaining that rhythmic behaviour maximised our chances of finding food and, at the same time, minimised our chances of becoming the food.

Light plays a crucial role in controlling our circadian rhythms. National Institute of General Medical Sciences

Interestingly, our circadian timing system has the ability to adjust to changes in the environmental light/dark cycle. Our hunter-gatherer ancestors had to be adaptable to the daylengths of the different seasons. These days our adaptive needs are a little different.

For example, when we travel across time zones we eventually become resynchronised. Every time daylight savings starts or stops, our circadian timing system shifts to the new schedule. The shift in the timing of our rhythms occurs as a response to the light signals changing.

Light is the strongest signal to our biological clock and is critical to both keeping us synchronised to the external environment, but also to resynchronising us when we land in a different time-zone.

Of course, the time it takes our system to shift can be uncomfortable – as evidenced by the symptoms of jet-lag – and hints at how critical it is to our well-being to be synchronised with the external environment.

But that brings us back to the question: do our rhythms drive us, or do we drive our rhythms?

In today’s 24-hour society there are multiple challenges to our ability to stay in time with the world. Long work hours, long study hours, night work, new babies, on-call work and technology are all common reasons why individuals might operate against their natural rhythms.

Approximately 1.4 million Australians are classified as shift workers but many more of us work long or irregular hours that also require us to be awake when our circadian timing systems would have us sleeping.

Significant challenges come with working during what our bodies perceive to be night. Reduced alertness, impaired performance, increased accident risk, negative impacts on cardiovascular and gastrointestinal health and psychological well-being are all examples of the consequences of fighting the rhythms that drive us.

But is it possible to use our circadian rhythms to our advantage? The short answer is yes, but it isn’t easy, and it’s all to do with light.

Let’s take the permanent night-shift worker as an example. Ideally, the night worker would be able to invert their rhythms so that their circadian system was driving them to be awake and alert during the dark hours and to sleep soundly during the daylight hours.

Night workers who are exposed to very little light actually do shift their rhythms and begin to look more like our nocturnal friends – they are awake and alert during the dark hours and sleep fairly well in the daytime. Studies focusing oil-rig workers in the North Sea show this very clearly.

But things are different if you work in a mine in northern Australia. Before you get into bed after each night shift you will probably see the sun come up, and that means your circadian system stays locked tightly to the day/night cycle. In other words, there is no shifting or adjusting of your rhythms.

You are a diurnal animal working a nocturnal schedule and your rhythms rule you. With 1.4 million people in this country working shift work, this is a big issue.

We aren’t programmed to be awake driving trucks or performing surgery or landing airplanes in the middle of the night. The question then is, what can be done to overcome the physiological tendency toward sleepiness and lowered alertness on the night shift? We’ve already talked about one of the answers – light.

The daily signal of light from the sun keeps most of us synchronised to our world. The other thing about light is that it is also an alerting signal. That means that light could be used to both increase alertness at night and also promote a shift in the circadian timing system.

Indeed, recent studies suggest that partial adjustment to night shift might have benefits for alertness and sleep without requiring complete shifts back and forth between day shift, afternoon shift and night shift. But can that help the night-shift worker? We think it can.

Researchers at Monash University, University of South Australia and the Woolcock Institute are recruiting night-shift workers to be guinea pigs in a study investigating whether special short wavelength light can improve alertness and performance on the night shift. If our results are positive, there may be applications beyond the night-shift worker.

Even day workers are driven by rhythms. Every day, just after lunch, we all go through a little concentration slump. Our core body temperature dips just a little which predisposes us to sleepiness, reduced alertness and sometimes impaired performance.

Perhaps one day we’ll be able to use light to our advantage to help ourselves out of that slump.

In the meantime, our circadian rhythms will continue to beat on.

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