We know that exercise can support our muscles and bones and practising mindfulness and meditation can help our mental health. 

But, when it comes to looking after your brain and body, how much attention do you pay to your circadian rhythm? Perhaps not a lot – yet it’s a crucial part of your wellbeing, influencing almost every process in the body.

A new paper, published in the Nature journal Translational Psychiatry, found that disruption to circadian rhythm was a common factor in those with mental health disorders, including anxiety and schizophrenia. The analysis of peer-reviewed studies didn’t establish cause and effect, but researchers said “the telltale sign of circadian rhythm disruption – a problem with sleep – was present in each disorder.”

What is your circadian rhythm?

“The circadian rhythm is generated by the master clock in the brain, an area of 50,000 cells near the hypothalamus [the hormonal control centre of the brain],” says Dr Kat Lederle, founder of sleep therapy platform Somnia. The master clock powers all of the cells in our body and influences many different processes and functions on a daily basis. “Your clock is set to a 24-hour rhythm, though sometimes it’s a little shorter or longer,” she says. 

Essentially, the circadian rhythm is crucial to establishing what our body and mind do over the course of a day. That helps us regularly get a good night’s sleep, which is crucial for supporting recovery from exercise, rebuilding muscle and energising us for our next workout. 

That might sound like a luxury not worth overthinking but a circadian rhythm that’s out of whack can also lead to exhaustion, mental health disorders and even put you at higher risk of illnesses and disease. After all, this clock does dictate how every single cell in our body works. “For our survival, it is crucial that our rhythms are in synchrony with the external world. For that, it needs daily time cues or ‘zeitgebers’ that work to reset the clock,” says Dr Lederle. 

There are five key zeitgebers that can support or interrupt our body clock: light, temperature, eating, movement and socialising. The first on that list is by far the most important.

1. Exposure to light (natural or otherwise)

A new study has been looking into why students tend to wake up later and go to bed later in the winter, and has found that the lack of daylight makes it infinitely harder to get up early. No surprises there. But we can mitigate the impact of dark mornings by ensuring our evening routine is geared towards sleeping earlier.

“Light during the day – especially in the morning – advances your clock, so you get tired earlier in the evening, but light exposure late in the day or early night will delay your clock, pushing back the time that you will feel tired,” explained the study’s author Horacio de la Iglesia. “Ultimately, the time that you fall asleep is a result of the push and pull between these opposite effects of light exposure at different times of the day.”

That’s something Dr Lederle agrees with: “When we open our eyes to light in the morning, it signals to the clock that the day has started and the clock will relay that message to all other cells in the body via cortisol. When it gets dark, the message that the day is ending is also passed around the body via melatonin. This is the most powerful and reliable way to shift the rhythm of your body clock,” says Dr Lederle.

The problem is that we’re not longer dependent on sunlight to dictate when it’s day and night. “For most of human history, we anchored our circadian rhythm to the sun’s brightness that increased throughout the day and reduced into the night,” says Dr Lindsay Browning, founder of Trouble Sleeping. “Now, there’s an ever-increasing number of other sources of intensive light and a 24/7 world can cause major disruption to our melatonin production and circadian rhythm.” 

It’s why avoiding screens before bed really can make a difference – but if that is impossible (or undesirable) then Dr Browning suggests lowering the level of blue light emissions (a more intensive form of light) with ‘night mode’ settings.

“The specific timing of light exposure can also be used to manipulate and shift our circadian rhythm. Bright light very early in the morning will help to move your bedtime and wake time earlier, helping you to wake up more easily in the morning and then fall asleep earlier at night,” she adds.

2. Morning exercise

The brain knows that we don’t move as much at night as in the day – in fact, in REM sleep stages the body is temporarily paralysed. For that reason, exercise can signal wake and sleep times to the body.

Dr Browning says that “some studies have found the timing of exercise can either advance or delay someone’s circadian rhythm. For example, exercising in the evening is shown to result in a phase delay (waking up later and going to bed later). Whereas, exercise in the morning was shown to phase advance (causing bedtime and wake time to move earlier in the day).” 

This could be because of exercise’s impact on blood pressure, which should naturally lower in the evening but spikes soon after movement. But it’s a two-way street, as researchers from a 2018 review into circadian rhythm and exercise found that “physiological functions associated with athletic performance have also been shown to follow a specific circadian rhythm”.

They suggested that our preferences for morning or evening exercise aren’t just to do with taste but also reflect when our circadian systems are at their peak as our body clock can vary our muscle’s flexibility, mental motivation and the impact of our nutrition. 

Researchers concluded that enhanced performance is most frequently seen in the early evenings in line with cortisol and testosterone fluctuations associated with our circadian rhythms, but that taking an individual’s own sleep-wake preferences can improve our workouts. And according to Dr Browning, the most important thing is maintaining a regular exercise routine, which means that your body knows what to expect and can factor it into its 24-hour rhythm. 

3. Eating earlier

“The circadian rhythm controls our ability to properly digest food, primarily through controlling the release of hormones such as insulin which increases as melatonin production decreases and vice-versa,” says Dr Browning. 

“That means we are not designed to digest food when our circadian rhythm thinks we should be asleep and we are less able to control blood sugar when eating during the night. In fact, shift workers who often have to eat during the night have a higher incidence of digestive issues compared to people who work a typical 9-5 day.”

That’s why eating at structured times is recommended to help with jet lag, and a good breakfast is shown to help properly anchor your day. Dr Lederle also adds that regular meal times impact our “peripheral clocks” – the second class of clocks that make sure cells comprising organs function as one. 

“The clocks in your liver and digestive system are, of course, impacted by eating. The issue is when organ clocks and the master clock behave as if they were in different time zones – it can cause a misalignment within the body which, over time, increases the risk of metabolic diseases,” she says. 

Dr Browning suggests sticking to regular and ‘traditional’ times (ie breakfast, lunch and dinner) to signal timings to your body clock and support your sleep cycle. 

4. Keeping cool at night (and sleeping naked)

“It is long established that our circadian rhythm controls our core body temperature. We have a slightly warmer body temperature during the daytime and as bedtime nears our body temperature starts to drop slightly,” says Dr Browning.

However, recent research published in Nature found that circadian clock neurons can be changed through environmental temperature, suggesting the temperature could be a cause rather than an effect of our circadian rhythms.

“That would make sense when considering that the sun (providing light and warmth) has been the primary influence of the circadian rhythm throughout human history. More research is needed to fully understand how we might be able to change our circadian rhythm by changing our ambient temperature, but generally, anything that maintains an elevated body temperature will interrupt your sleep – that’s obvious if you’ve ever struggled to sleep during summer as your environment doesn’t cool down,” Dr Browning adds. 

It doesn’t only impact us falling asleep, but also how restorative our sleep is: hot temperatures have been shown to pull us out of deep sleep, the stage in which our bodies are most likely to repair and recover from exercise.

To keep cool at night, she recommends breathable bed sheets and avoiding turning up the heating high at night (no matter how tempting it is in the colder months). Sleeping naked has also been associated with better temperature regulation and more deep sleep and REM stages, according to a 2012 paper. 

“It may seem counter-intuitive at first but a warm bath can actually help you get to sleep – this is because the bath temporarily and artificially elevates your body temperature and, as you cool, your body mimics the natural lowering of your temperature to make you feel increasingly sleepy,” says Dr Browning.

5. External interaction

Our social connection, workflow and general busyness can also have an impact on how our circadian rhythm works. “If our work-sleep schedule is different to the rhythm set by our body clock and varies between the week and weekend then it can have a knock-on impact,” says Dr Lederle.

And studies have shown that when we increase our social connection (such as by going out to meet people or socialising) we can modify our circadian rhythm to stay awake longer.

“However, it may not be the social connection itself that is affecting our circadian rhythm, but rather the fact that as we meet people and socialise we are likely to experience increased light exposure in these social settings, compared to staying at home,” says Dr Browning. Then there’s the impact of alcohol, which can potentially shorten our circadian rhythms, suggesting that the body clock will be in a push-pull state when we stay out late and drink. 

Unfortunately, our nine-to-five, five-days-a-week world isn’t really set up for routine work and socialising patterns. Perhaps, where possible, we can maintain interactions on the weekend to the same time frame that we work in during the week – though that isn’t possible for all.

As Dr Lederle points out: “All these zeitgebers influence each other and either push or pull in the body clock. That’s why regularity is so important. Regularly timed exercise, meal-times, interaction and, most crucially, light exposure are what help your body to stay set on the same circadian rhythm.

“Ideally, the rhythm that we are trying to impose is the same our body clock is naturally set to. Some people are morning people, others are late, and the rest are somewhere in between. You can tweak the clock with daily habits but you need to find what works for you. If we organise our day and activities to match our biological rhythm then that’s best for our health and wellbeing.” 

Images: Getty