NOW well into its second year, the COVID-19 pandemic continues to put the mental and physical health of Australians at risk.

The “lockdown” or “stay-at-home” strategy has emerged worldwide as one of several effective means of interrupting the spread of COVID-19 through our communities. But it is now clear that protracted periods of lockdown have had a negative impact on our collective mental health, with one contributing factor being the disruption of our established rhythms of life.

With the pandemic and lockdowns continuing for a while, it is timely to speak about how our daily rhythms and mental health can be affected, and to discuss practical strategies to keep our “body clocks” healthy during the pandemic and beyond.

Circadian rhythms and health

We live on a planet that spins on its axis, completing a full rotation every 24 hours, creating our day and night. Millions, if not billions of years ago, evolutionary pressures favoured organisms that developed a biological “clock”, a timepiece that tracked the daily rising and falling of our sun.

In 2021, virtually every living organism contains a network of biological clocks that run to approximately 24 hours, known as circadian rhythms or circadian clocks. For us mammals, the so-called master clock resides in a small bundle of cells at the base of our brain called the suprachiasmatic nucleus. Its main source of information about time comes from light.

We humans benefit immensely from our circadian system – it regulates the timing of virtually all processes necessary for health and wellbeing, including our sleep-wake cycle, mood, energy, cognition, metabolism, digestion and immunity. For these reasons, the new field of circadian medicine is taking centre stage in our understanding of health and illness.

Importantly, just as man-made clocks can come out of time, so too can our circadian clocks. Most of us are familiar with the experience of jet lag — a syndrome caused by the mistiming of our inner biological time and the outer time of the local day and night cycle — brought on by rapid travel across time zones. Our circadian system can be perturbed by a range of other factors such as infection, inappropriately timed light exposure, and irregular sleep-wake routines, and can lead to a constellation of mental and physical phenomena like fatigue, mood disturbance, irritability, and gastrointestinal problems, among others. Even more, circadian disturbance is now recognised as a risk factor for a range of major health conditions including stroke, cardiovascular disease, mood disorders, and dementia (here and here).

The “circa” in circadian means “approximately”. This reflects that our circadian system produces internal rhythms that are approximately 24 hours, but not precisely 24 hours. Like a clock that runs a bit fast or a bit slow, it must be reset each day. Exposure to light – especially from the morning sun – is the most important determinant of the regularity of our circadian rhythms (here and here). Other important but far less potent factors include the regularity of sleep-wake schedules and the timing of physical activity and possibly social activity.

Critically, each of these factors involved in the regulation of our circadian clocks is likely to have been affected during the pandemic and especially during lockdowns.

Circadian rhythms in the pandemic and lockdown

Data during the COVID-19 pandemic are now available from many countries around the world reporting changes in behavioural and biological rhythms, as well as alterations in factors that are critical for the healthy functioning of our circadian system.

To begin with, several large survey-based studies have reported some positive changes in the sleep-wake cycle during periods of lockdown (here and here).

For example, a survey of over 7000 adults from 40 countries who underwent a month-long period (on average) of social restrictions due to COVID-19 reported an increase in sleep duration on weekdays and a decrease in sleep duration on weekends. Alongside these changes in sleep-wake timing, there was a reported reduction in “social jetlag”, which refers to the discrepancy between sleep times on workdays versus work-free days.

Similarly, a study of approximate sleep timing from a sleep-tracking smartphone application from over 8000 users in the US, UK and South Korea reported a 13-minute increase in estimated sleep duration between March 2019 and March 2020, and a 22-minute increase in estimated sleep duration between April 2019 and April 2020.

Together, these studies suggest that a large population-level sleep deficit present before the pandemic may have been partially relieved in some people by a relaxing of how we spend our social time (eg, shifting work schedules), and emphasises the need to think carefully about how we navigate our schedules as we come out of the COVID-19 pandemic.

Importantly, several studies also show negative changes during the pandemic and lockdowns related to the circadian system.

For example, one survey study of about 800 health care workers in the US collected data on changes in a range of lifestyle factors (including work, sleep, screen time) and reported that following COVID-19 stay-at-home orders, people’s self-reported mood became worse, exercise decreased, and substance use and screen time increased. Compared with the survey respondents who indicated that their mood stayed the same or was better during the pandemic, respondents whose mood became worse reported that after stay-at-home orders they were sleeping less, were less likely to have had their sleep time shift later, were more likely to be working from home, and were more likely to be using screens more before bed.

Similarly, a large survey of individuals in the US and Australia reported significant disruptions in social life, family life, work and study, sleep, sexual activity, and physical activity, with more time spent on screens (possibly increasing inappropriate night-time light exposure) and less times commuting (probably decreasing bright and regular day-time light exposure).

Altogether, these studies show a range of lifestyle changes that indicate disruption in factors related to the circadian system (light, physical activity) which may partly explain some of the population-level changes in mood and mental health during the pandemic and highlight important areas for education and lifestyle alteration related to circadian health.

Some of these considerations are likely to be of high relevance to certain groups of people who may be more sensitive to the effects of circadian disturbance; for example, people with mood disorders such as depression or bipolar disorder (here and here). Importantly, we know of a range of simple lifestyle modifications that people can implement in their daily lives to maintain the functioning of their sleep-wake cycle and circadian system, both during lockdown and beyond.

Lifestyle strategies for circadian health

Light exposure

Light is the most important signal that tells our circadian system what time it is. Circadian rhythms developed to track and predict the 24-hour light/dark cycle. Bright light in the morning (ie, sunlight) is extremely important for “resetting” our internal clock each day. Conversely, light in the evening hours (from screens or electric home lighting) can suppress the sleep-promoting hormone melatonin and delay our clocks, making it harder to fall asleep and wake rested. Rethinking our relationship with light can help our circadian clocks function in a more natural and healthy way.

People should try to maximise exposure to sunlight during the day – especially in the morning – for example, by taking a morning walk outside or sitting by a window after waking up. To reduce the negative effects of light on your circadian clocks, try to eliminate screen-time and other sources of light 1–2 hours before bed – try swapping time on screens for reading a book or listening to music or a podcast. Be aware that our clocks expect bright days and dark nights. The closer you keep to what your clock expects, the better your sleep and general health will be.

Sleep-wake cycles

The time that we go to sleep each night and wake each day strongly influences the timing of our circadian clocks. It’s important that we try to keep a regular sleep-wake schedule and follow it during the week and on weekends. You should follow whatever regular schedule feels natural to you (and leaves you feeling your best), even if it differs somewhat from others in your household.

Physical activity

Regular physical activity is thought to maintain strong circadian clocks. Try engaging in exercise each day at around the same time; however, don’t exercise too late in the evening as this raises core body temperature and can make it harder to fall asleep and wake rested.

Social interaction

It’s more important than ever that we stay connected. While we can’t meet with friends and family now in familiar ways, try to schedule regular times to call or exercise with friends of family throughout the week, which may help regulate our circadian rhythms.

Meal timing

We are learning more and more about how when we eat influences our physical health. The timing of our meals influences the timing of circadian clocks located in organs like our stomach and liver, and eating haphazardly throughout the day, and too close to bed, might have negative effects on how we metabolise and digest food. Try to eat meals at a regular time each day and eat your last bite approximately 2 hours before you go to bed.

In closing, while the COVID-19 pandemic has forced us all to reconstruct our daily lives, it has also allowed some opportunity to reflect on our lifestyles and how we might live better. Learning about and changing our relationships with our circadian clocks is one way to improve all aspects of our health and wellbeing.

Jacob Crouse is a research fellow at the University of Sydney’s Brain and Mind Centre. His work investigates the links between circadian rhythms and mood disorders in young people.

Joanne Carpenter is a research fellow at the University of Sydney’s Brain and Mind Centre. Her work focuses on biological circadian rhythms and mental health in young people.

Sean Cain is an Associate Professor at Monash University and is the current President of the Australasian Chronobiology Society. His work focuses on how light impacts sleep and mental health.

Ian Hickie is Co-Director (Health and Policy) and Professor at the University of Sydney’s Brain and Mind Centre. His work focuses on developing real-time personalised and measurement-based care systems in youth mental health.



The statements or opinions expressed in this article reflect the views of the authors and do not represent the official policy of the AMA, the MJA or InSight+ unless so stated.

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