Circadian Rhythms and Health: A Comprehensive Overview

Introduction

Circadian rhythms (from the Latin circa diem, “about a day”) are natural 24-hour cycles in the body that align our biology with the day-night cycle (1). Nearly every living thing, from humans and animals to plants and even microbes, has internal clocks that drive daily changes in behaviour and physiology. These rhythms allow us to anticipate daily environmental changes (like light and darkness) and adjust our bodies accordingly (2, 3). In humans, circadian rhythms orchestrate crucial functions such as the sleep-wake cycle, hormone release, body temperature, metabolism, and even mood and cognition (4). Maintaining synchrony between our internal clock and the external world is vital: when our internal rhythms fall out of sync with the 24-hour day (for example, due to shift work, travel, or irregular schedules), it can seriously impact our health and well-being (1, 5).

The human circadian biological clock influences many daily patterns. For instance, the sleep hormone melatonin rises in the evening (promoting drowsiness) and peaks during the night, while the stress hormone cortisol spikes in the early morning shortly after waking to promote alertness (6). Core body temperature also oscillates each day, it drops at night as bedtime approaches and rises toward midday (7). Even physical performance follows a rhythm (many people have their fastest reaction times and best coordination in the afternoon). By aligning our sleep, meals, and activities to these natural highs and lows, we can optimise energy, mood, and overall health.

Biological Mechanisms of Circadian Rhythms

The Suprachiasmatic Nucleus: Master Clock in the Brain

Deep in the brain’s hypothalamus is a tiny region called the suprachiasmatic nucleus (SCN), this is the body’s master circadian clock (1). The SCN is a cluster of about 20,000 neurons situated right above the optic chiasm in the anterior hypothalamus (8). This central clock generates a near-24-hour rhythm and coordinates circadian cycles throughout the body. Landmark experiments showed that if the SCN is damaged, normal daily rhythms disappear, and transplanting a healthy SCN can restore those rhythms (8).

The SCN’s timekeeping is endogenous, it maintains roughly a 24-hour cycle but needs daily adjustment (4, 9). In most adults, the intrinsic period of the SCN’s clock is slightly longer than 24 hours (about 24.2 hours on average) (4). To stay aligned with the Earth’s 24-hour day, the SCN relies on environmental cues. The most powerful of these timing cues (known as zeitgebers) is the light-dark cycle (7). Other zeitgebers include regular mealtimes, physical activity, and social routines (10). The SCN receives light input via the eyes and calibrates our internal clock to the day-night cycle.

Light Exposure and the Day-Night Signal

Light is the master synchroniser of circadian rhythms. Specialised cells in the retina send signals directly to the SCN via the retinohypothalamic tract (7). Morning light activates the SCN, adjusts hormone levels, and prepares the body for activity. Morning light also suppresses melatonin and boosts metabolism (7). Conversely, darkness at night triggers melatonin secretion, promoting sleep (6). Even dim evening light from screens or artificial lighting can suppress melatonin and delay sleep onset (11). Humans evolved with bright days and dark nights, artificial lighting at night can confuse our clock (11, 12).

Hormone Rhythms: Melatonin and Cortisol

Two key circadian hormones are melatonin and cortisol:

• Melatonin, the “sleep hormone,” is released by the pineal gland in response to SCN signals. It begins rising in the evening, peaks during the night, and drops in the morning (6).

• Cortisol, a hormone related to alertness and metabolism, is lowest overnight and peaks in the early morning (6, 13).

  
  

These hormones regulate the sleep-wake cycle: melatonin promotes sleep, while cortisol promotes wakefulness. Disrupting circadian timing shifts the release of these hormones, leading to grogginess or insomnia.

Peripheral Clocks Throughout the Body

Besides the SCN, peripheral clocks exist in almost all body tissues, including the liver, heart, and digestive system, and are regulated by clock genes (14). These local clocks follow their own rhythms but are synchronised by the SCN using temperature, hormones, and behavioural cues (14). Misalignment between the SCN and peripheral clocks (for instance, from irregular eating or sleep patterns) can cause internal desynchrony, leading to health problems.

Health Consequences of Circadian Disruption

Sleep Problems and Disorders

Circadian misalignment often leads to insomnia, irregular sleep, and disorders like shift work disorder (15, 16). The wrong signals at night (e.g., artificial light) can delay melatonin release, preventing restful sleep and causing chronic sleep deprivation (1).

Metabolic and Cardiovascular Issues

Disrupted circadian rhythms are linked to obesity, type 2 diabetes, and cardiovascular disease (17, 18). Eating at the “wrong” time, like late at night, worsens blood sugar control and leads to weight gain (17). Shift workers are at higher risk of metabolic syndrome and heart disease (18). The WHO classifies chronic shift work as a probable carcinogen due to circadian disruption (17).

Mental Health and Mood

Circadian disruption contributes to depression, anxiety, and bipolar disorder (19). People with irregular sleep schedules or “night owl” tendencies are at higher risk of mood disorders. Regular light-dark exposure and consistent routines improve emotional regulation (16).

Cognitive Performance and Productivity

When circadian rhythms are off, cognitive abilities decline. Reaction time, focus, and memory suffer, especially during “biological night” hours (20). Misaligned workers show more errors and are more accident-prone due to decreased alertness (16, 20).

Immune Function and Inflammation

The immune system follows a daily rhythm. Circadian disruption reduces immune efficiency and increases inflammation (21). Studies show irregular sleepers catch colds more often and respond worse to vaccines (21).

Hormonal Imbalances

Disrupted rhythms alter levels of ghrelin, leptin, insulin, and reproductive hormones (17). This leads to increased appetite, poor glucose control, fertility issues, and flattened hormonal cycles (17).

Benefits of Aligning with Natural Rhythms

Realigning your body clock provides:

• Better Sleep – improved melatonin release, fewer wakeups (6, 7)

• Higher Energy – improved morning cortisol and alertness (13)

• Healthier Metabolism – weight control, better insulin sensitivity (17)

• Improved Mood – reduced depression and anxiety (19)

• Enhanced Immunity – better pathogen defence, healing (21)

• Balanced Hormones – stable appetite and stress response (6, 17)

  
  

Practical Strategies for Optimising Your Circadian Rhythm

1. Morning Light Exposure – go outside early to reset your SCN (7).

2. Consistent Sleep Schedule – regular bed/wake times reinforce rhythms (1, 15).

3. Evening Light Management – reduce screen use, use dim/warm lights (11, 12).

4. Use of Temperature Cues – cool room at night, warmth in the morning (7).

5. Time Your Meals – eat earlier in the day; avoid late-night eating (17).

6. Exercise Timing – morning/afternoon workouts align better than late-night (22, 23).

7. Caffeine and Stress – avoid both late in the day to support sleep (15).

8. Melatonin/Light Therapy (if needed) – especially for shift workers or jet lag (1, 24).

Conclusion

Circadian rhythms govern every major function in the body. Aligning with natural cues, such as daylight, meal timing, and rest, can improve sleep, mood, metabolism, hormone regulation, and immune health. In contrast, disruption leads to a cascade of problems. Fortunately, the solutions are simple, free, and rooted in nature: sleep regularly, see the morning sun, dim your lights at night, eat and move consistently. Living in sync with your internal clock is one of the most powerful things you can do for your health.