What Happens When Circadian Rhythms Are Disrupted? How Light Patterns Affect Health

Up late at night and sleeping late into the morning? New research published in the journal Proceedings of the National Academy of Sciences, suggests daily patterns of light and dark exposure, in particular a lack of daylight and too much light at night, can disrupt our natural circadian rhythms, significantly impacting health and increasing the risk of mortality.

Light Exposure and Mortality Risk

The study by researchers from FHMRI Sleep Health, explored whether personal patterns of day and night light exposure could predict mortality risk.

All up, the researchers looked at 13 million hours of light exposure data from 89,000 UK Biobank participants aged between 40 and 69 years. Participants were followed over eight years, with the researchers tracked sleep duration, sleep efficiency, and midsleep using motion data from wrist-worn sensors, while cardiometabolic mortality was defined as any cause of death related to diseases of the circulatory system or endocrine and metabolic diseases.

So what did the research find?

• Individuals exposed to high levels of night-time light experienced a 21-34% increased risk of death.

• In contrast, those enjoying high levels of daylight saw a 17-34% reduction in their mortality risk.

The relationship of both day and night light exposure with mortality risk held even after adjusting for other factors, including age, sex, ethnicity, photoperiod, socioeconomic advantage, physical activity, social activity, smoking, alcohol, urbanicity, shift work, and baseline cardiometabolic health.

Interestingly, sleep did not fully account for the light and dark exposure relationships either.

Short sleep, which could lead to being up for more hours at night, did account for some, but not all, of the increased mortality risk from nighttime light exposure. Further, it didn’t decrease the benefit from daytime light exposure.

On the other hand, long sleep or disrupted sleep did not change the relationship of night and day time light with mortality risk.

Taken together, the researchers suggest that human circadian rhythms (not sleep per se) and the effects that light exposure patterns have on the body clock are key to these findings.

Senior author and sleep expert Professor Sean Cain from Flinders University highlights the significance of these findings:

"Exposure to brighter nights and darker days can disrupt our circadian rhythms, a disruption that is known to lead to various health issues including diabetes, obesity, cardiovascular disease, mental health issues, and increase the risk of death."

What Are Circadian Rhythms, and Why Do They Matter?

You might be curious, what exactly are circadian rhythms, and why are they so important?

Circadian rhythms are essentially the body's internal clock, regulating the sleep-wake cycle and other physiological processes over a 24-hour period. Light is a primary cue for these rhythms, helping to synchronize our internal clock with the external environment. Activity and diet are secondary cues.

Associate Professor Andrew Phillips, co-senior author of the study, explains that night-time light exposure disrupts circadian rhythms by shifting their timing (phase-shift) and weakening their signal (amplitude suppression).

This disruption can lead to adverse health outcomes, particularly affecting the central circadian 'pacemaker', which orchestrates rhythms throughout the body.

"Disruption to the body's circadian rhythms is linked to the development of metabolic syndrome, diabetes, and obesity and is also strongly implicated in the development of cardiometabolic diseases including myocardial infarction, stroke, and hypertension,"

How Can We Use This Information?

Lead author Dr. Daniel Windred emphasizes the importance of maintaining a dark environment across late-night and early-morning hours (1am - 4am) when the central circadian 'pacemaker' is most sensitive to light. Conversely, seeking bright light during the day can boost circadian rhythms.

"Protection of lighting environments may be especially important in those at risk for both circadian disruption and mortality, such as in intensive care or aged-care settings,"

Added Associate-Professor Phillips:

"Our findings clearly show that avoiding night light and seeking daylight may promote optimal health and longevity, and this recommendation is easy, accessible, and cost-effective,"

Reference

Daniel P. Windred, Angus C. Burns, Jacqueline M. Lane, Patrick Olivier, Martin K. Rutter, Richa Saxena, Andrew J. K. Phillips, Sean W. Cain. Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals. Proceedings of the National Academy of Sciences, 2024; 121 (43) DOI: 10.1073/pnas.2405924121

Clinical Comment

This research highlights the hidden health risks from strong patterns of evening light exposure and limited daytime light exposure. Given that they accounted for shift work and the pattern still held, this suggests that those with non- shift work circadian rhythm disorders are especially at risk.

Given the age range of study participants this study does not comment on health risks for another group that is likely to have patterns of increased dark exposure, and decreased light exposure: teens and those in their early twenties. But other research suggests this pattern in younger people is linked to poor mental health.

In terms of overall health, it signals the importance of considering the “amplitude” or strength of one’s circadian rhythm in health. What is circadian rhythm amplitude? Circadian amplitude is the difference between the peak and trough of a circadian rhythm. The bigger the difference, the stronger the circadian rhythm and the better one’s physical and mental health (generally speaking).

Lower troughs will be related to being asleep during the nighttime (i.e. in darkness). Higher peaks would be associated with more light exposure and higher levels of daytime activity.

A flat circadian amplitude would be associated with:

• Little activity across day and night, or

• A lot of activity across both day and night, and

• Patterns of little daytime activity and lots of nighttime activity (this is because the circadian rhythm naturally peaks during the day, and bottoms out during the night, so inverting this pattern will flatten the amplitude).
  

This also reinforces what we are often telling clients: lying in bed trying to “catch up” after a difficult night will simply disrupt circadian rhythms and create new problems (including contributing to daytime fatigue). Keeping a regular wake time and staying active during the day will offset some of the impact of a difficult night and support overall health.

Key takeaways from our perspective?

Embrace Natural Light: Aim to get plenty of daylight exposure, particularly in the morning, to help synchronize your circadian rhythms. Other research suggests that increased daylight exposure stabilizes the circadian phase (the timing of your rhythm), enhances circadian amplitude, and reduces sensitivity of the circadian system to night light.

Focus On The Morning: The earlier you rise, the earlier you will fall asleep (over time). This means the morning needs to be the key focus, not the evening. Focus on keeping the morning consistent and the evenings will come.

Consider Treatment: For those with disrupted circadian rhythms, such as delayed sleep phase disorder or shift work disorder, light therapy and other treatments can be a useful tool to reset your internal clock.

You can get in touch or book an appointment directly.

We don’t just work with insomnia, we also treat and support other sleep disorders including circadian rhythm sleep disorders, restless legs syndrome, and parasomnias such as nightmare disorder. We don’t treat obstructive sleep apnea but can provide info and direction on providers if needed  

Article Author: Dan Ford Sleep Psychologist