The Rhythm Within: How Circadian Timing Matters for Health and Mood

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The first time I heard about circadian rhythms was in my high school biology class. I remember breezing past the term, memorising that “circadian rhythms = sleep-wake cycles,” and not thinking much more about it. That is, until I started to study them in graduate school.

As scientists have uncovered more about circadian rhythms, we now realise how deeply they affect our health and daily lives. Circadian rhythms are the body’s hidden timekeepers, quietly influencing when we feel alert, hungry, or even happy. In fact, the study of time, known as chronobiology, has blossomed into interdisciplinary fields such as chronopharmacology/chronotherapy, which examines how the timing of drug administration and therapeutics can change their efficacy and side effects, and chronopsychology, which explores how timing influences psychological processes like memory, perception, mood, and performance.

Building on this emerging science, my research explores how circadian timing shapes the brain. I am a PhD student in Dr Laura Fonken’s laboratory at The University of Texas at Austin, where I study how the body’s internal clock interacts with the brain’s immune system to influence mood and behaviour. This article explores the different factors that influence circadian rhythms and some strategies to improve circadian health.

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What is The Circadian System?

The word circadian, coined by Dr Franz Halberg in 1959, derives from the Latin words circa (“around”) and dies (“day”), meaning “approximately a day.” Circadian rhythms are the body’s approximately 24-hour clock that helps maintain alignment with the Earth’s day and night. Over time, this internal timing evolved to help living organisms anticipate and adapt to the predictable daily changes in their environment. Essentially, circadian rhythms optimise bodily processes by using and conserving energy at the appropriate times.

Observations of daily biological rhythms date back as far as the 4^th century BCE, when the Greek philosopher Theophrastus noted that tamarind tree leaves opened at sunrise and closed at night. Nearly 2,000 years later, advances in DNA sequencing revealed that a network of genes controls these daily patterns. Researchers named the gene network the “molecular clock” and discovered that it runs automatically within almost all living organisms – from bacteria to plants to animals, including humans.

Remarkably, the molecular clock keeps running without external cues, like light. For example, in studies where participants spend weeks in caves without access to natural light, they continue to wake up at consistent times each day, guided by their internal clock rather than sunlight. In another study, volunteers lived in bunkers unaware of day or night, yet their body temperature, activity levels, and even urine production maintained a roughly 24-hour rhythm (the scientists explain this in more detail here!). These studies proved that our sense of time is not merely set by the environment; it is hard-wired into our biology.

However, just like a mechanical clock, the circadian system can drift because the internal rhythm is not perfectly 24 hours long. To stay on track, it needs regular “resets” or resynchronisations through environmental cues like light, food, and social activity.

Light: The Most Influential Timekeeper

While circadian rhythms repeat on their own, there are synchronisers in our environment that act as time cues known as “zeitgebers”. The light/dark cycle is the most important zeitgeber. When light enters the eyes, it sends information to a small brain region called the suprachiasmatic nucleus (SCN), which acts as the body’s “primary clock.” The SCN then helps coordinate clocks throughout the brain and body. Viewing morning sunlight helps “jumpstart” the circadian system.

Does this mean staring directly at the sun is good for you?

No. However, getting natural light early in the day (even just 10 minutes, though 30 is even better!) is extremely beneficial. Even on cloudy days, outdoor light can still be far brighter than indoor light, signaling your brain to wake up and be alert.

When light is perceived after sunset, it disrupts this synchronisation. The SCN interprets nighttime light as daytime, disrupting the molecular clock and suppressing melatonin, a hormone that promotes sleep. Light signals can also reach other brain regions involved in mood and anxiety regulation. Over time, repeated misalignment can desynchronise internal systems, leading to impaired alertness, mood disturbances, metabolic dysregulation, and increased risk for chronic conditions such as obesity, diabetes, and depression.

In this way, light acts both as a timekeeper and a potential disruptor: when timed correctly, it reinforces healthy circadian rhythms; when mistimed, it confuses the body’s sense of day and throws bodily processes out of sync.

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Feeding the Clock: Food as Another Timing Cue

Food is another timing cue for the body. Regular meal timing ensures that digestion, metabolism, and energy use occur at biologically appropriate times. It also ensures that the body can rest and repair during the fasting phase.

When food intake is irregular or occurs late at night, it can send conflicting signals to the body. Eating at all hours of the day has also been linked to a host of problems, including increased risk of obesity, insulin resistance, heart disease, and chronic inflammation. Approaches like intermittent fasting (or time-restricted feeding in animal research) leverage circadian rhythms by limiting food intake to a consistent window each day, often within 8-12 hours during wakefulness. 

Interestingly, recent research has shown that when we eat is almost as important as what we eat. One study showed that mice fed a high-fat diet under time-restricted feeding were protected from obesity, excess amounts of insulin in their blood, and inflammation even though they consumed the same number of calories as mice that ate freely throughout the day. Furthermore, time-restricted feeding may also improve mood: my research from the Fonken Lab showed that time-restricted feeding improved sociability and reduced behaviours associated with anxiety in mice. While these studies were conducted in animals, growing evidence suggests that humans benefit from intermittent fasting as well.

So what does this mean? For humans, consistent, daytime-aligned feeding schedules can help reinforce circadian rhythms and promote health, whereas irregular or nighttime eating patterns can desynchronise the body clock and impair normal bodily functions.

Connection and the Clock: Social Life Shapes Timekeeping Too

Nearly every person has an internal timing preference. Some are “early birds”, while others are “night owls.” This natural variation influences when we feel most alert or tired, and problems arrive when social schedules (like school or work start times) don’t match our internal clocks. This mismatch, called social jetlag, can leave us feeling perpetually tired and out of sync.

But, social jetlag is not just about feeling groggy. It reflects a chronic misalignment between biological and social time that can affect mood and long-term health. Like irregular light exposure and eating habits discussed above, social jetlag is linked to high risks of obesity, diabetes, depression, and heart disease. It tends to peak in young adulthood, when biological clocks naturally run late, and often declines with age as people gain more control over their schedules. It is also common among “night owls,” whose natural rhythms clash with early work schedules.

The good news: when social and biological time align, people usually experience better sleep, higher energy, and improved mood and performance. Regular exercise, shared meals, and consistent social routines act as beneficial time signals that reinforce the body’s natural rhythms, supporting metabolic and mental health.

Keeping Rhythms in Sync: Strategies for Circadian Health

We’ve briefly touched on three factors that can influence our circadian rhythms, though many others such as physical activity, stress, and genetics also play significant roles. Modern life has decoupled human routines from natural day-night light cycles, but small, intentional changes can help realign our internal clocks. Prioritising morning light exposure, maintaining regular sleep and meal times, limiting bright light (especially screens) before bed, and building consistent daily routines all support circadian health. When our internal rhythms are in sync with the environment, we tend to sleep better, think more clearly, and feel more energised throughout the day!