The Effects of Circadian Rhythm on Physical and Mental Health

Circadian rhythm, often called the “body clock,” regulates nearly every aspect of our health. When it’s thrown off balance by inconsistent waking and sleep schedules, lack of natural light or chronic sleep disturbances, the negative effects can impact physical and mental health. Understanding the impact that circadian rhythms have on your clients is essential for a whole-person approach and lasting behavior change.

What is Circadian Rhythm

Circadian rhythms are 24-hour biological cycles that govern sleep-wake patterns, hormone release, digestion, immune function and cellular repair. This is our body’s operating system, and everything feels smooth when it runs on schedule. However, when disrupted, “glitches” begin to appear in nearly every system.

These rhythms are controlled by both central and peripheral “clocks” and are synchronized by environmental cues, primarily light.

Links to Health Outcomes

Our circadian rhythm is connected to many important health outcomes:

• Physical health: Circadian disruption is strongly linked to metabolic syndrome. Poor sleep and sleep disorders alter glucose metabolism, increase nocturnal cortisol secretion and contribute to poor dietary choices and greater caloric intake, all of which are drivers of obesity and cardiometabolic disease.
• Mental health: A strong connection also exists between circadian rhythms and mental health. Mood disorders often coincide with disrupted circadian–controlled responses, including sleep quality and cortisol regulation. Jet lag, nightshift work or artificial light at night can worsen depression and anxiety symptoms in susceptible individuals.
• Cognitive and immune function: In addition to sleep and mood, circadian disruption affects cognition and immune response. Studies note impairments in attention, memory and occupational performance, as well as reduced immune resilience.
• Performance: In athletes and tactical populations (e.g., military personnel, firefighters and law enforcement officers), circadian misalignment impairs reaction time, impairs recovery and heightens injury risk. Even one night of poor sleep can impair insulin sensitivity and diminish training quality the following day.

A Strong Predictor of All–cause Mortality

Unlike metabolic syndrome, which reflects downstream consequences such as high blood pressure and high blood sugar, circadian disruption operates upstream, influencing the very systems that regulate those outcomes.

Disruption accelerates biological aging, impairs deoxyribonucleic acid (DNA) repair and drives metabolic dysfunction. Circadian misalignment is emerging as a stronger predictor of all-cause mortality than metabolic syndrome.

Who Might Be Most Affected

Populations that might be most affected by circadian disruptions include individuals who work overnight shifts, older adults and people with sleep disorders.

Healthcare providers, first responders, flight crews/frequent flyers and others whose schedules consistently clash with natural light/dark cycles may have an increased risk of environmentally driven circadian and sleep disruption.

Older adults often experience age-related changes in sleep architecture (i.e., the structure and pattern of sleep cycles) that make circadian rhythm regulation more difficult. They often have shorter sleep duration, more fragmented sleep and a reduction in deep, restorative sleep. The body’s ability to respond to light cues also weakens with age, so the circadian system has a harder time “resetting” each day.

Conditions such as insomnia or sleep apnea dramatically affect restorative sleep. Sleep apnea may be compounded by adverse effects of dietary choices.

Why Light Exposure Matters

Compared to those who live in natural environments or spend a lot of time outdoors for work, many individuals are exposed to about four times less light during the day and twice as much artificial light after sunset. These shifts are linked to higher obesity prevalence across more than 80 countries.

Light is the strongest time cue for circadian rhythms. Morning sunlight exposure helps anchor the clock, while evening light, especially blue light from screens, delays it.

Epidemiological research shows strong associations between nighttime illumination and obesity prevalence worldwide. The good news is that even simple adjustments, like 10 to 20 minutes of outdoor light exposure in the morning, can significantly improve circadian alignment.

Support Clients with Practical Strategies

For health and exercise professionals, supporting circadian alignment can be part of a broader client-centered approach. Help clients set small, realistic goals like walking outside in the morning, taking lunch calls outside, going for a lunchtime walk or dimming lights after dinner. Acknowledge individual barriers like shift work, family obligations or sleep disorders, and tailor strategies accordingly, referring out when necessary. Lastly, pair exercise programming with sleep hygiene (i.e., habits and practices that promote healthy sleep patterns and improve sleep quality), light management and stress management for maximum impact.

Share the following strategies with your clients to improve sleep hygiene:

• Maintain a consistent sleep/wake schedule, even on weekends, if possible.
• Get natural light exposure in the morning.
• Minimize bright or blue light at night. Add “warm” light lamps to your home instead.
• Time meals to take place during daylight hours and avoid late-night eating, as sleep restriction increases energy intake and worsens body-composition regulation.
• Limit caffeine to the early part of the day, preferably to before 2:00 p.m.
• Build movement into daily routines, ideally earlier in the day.

Final Thoughts

Circadian rhythm disruption is more than a sleep problem. It’s a systemic health challenge tied to physical disease, mental health, immune dysfunction and shortened lifespan. Studies suggest it may surpass metabolic syndrome as a predictor of all-cause mortality, making it a critical area of focus for health professionals.

By helping clients align their daily habits with their body clock through sleep consistency, strategic light exposure, mindful nutrition and regular movement, you can enhance your clients’ well-being, improve performance and support goals and longevity. Circadian health isn’t just about better rest—it’s about building a foundation for lasting health.