The importance of circadian biology on health

Circadian biology is the science of how your body keeps time across a 24‑hour day–night cycle, orchestrating everything from mitochondrial energy production to immune surveillance, hormone pulsing, digestion, mood and cognition. When we live out of sync with these rhythms through artificial light, indoor living, erratic sleep and irregular meal timing our internal clocks drift away from nature’s cycles, and health inevitably pays the price.

Your body’s internal clock network

Deep in the brain you have a structure called the hypothalamus and within this sits the suprachiasmatic nucleus (SCN), which is often referred to as your master clock. The SCN receives light signals from the eyes and synchronises almost every organ system to the external light–dark cycle. Each tissue liver, gut, heart, immune cells, fat tissue and even skin has its own local “peripheral” clocks, built from molecular feedback loops of clock genes such as CLOCK, BMAL1, PER and CRY, which oscillate over roughly 24 hours and time local functions like detoxification, insulin sensitivity, DNA repair and inflammatory tone.

When morning light hits specialised melanopsin‑containing retinal cells within the eyes, a signal travels via the retinohypothalamic tract to the SCN in the brain, which effectively tells the body “it is daytime,” coordinating cortisol release, a rise in body temperature, increased alertness, and downstream timing cues to peripheral clocks. At night, darkness and dim, warm light allow the pineal gland to release melatonin, and melatonin then feeds back to the SCN and peripheral clocks, reinforcing night‑time programmes of antioxidant defence, mitochondrial repair, immune recalibration and autophagy.

Figure 1 | The Suprachiasmatic Nucleus – image from https://www.epilepsyinenglish.ie/clocks/

The 24‑hour rhythm map

Rather than thinking of circadian biology as only “sleep and wake,” it helps to picture a 24‑hour choreography where different systems step into and out of the foreground across the day and night. Between about 21:00 and midnight, melatonin begins to rise, core body temperature falls, and the nervous system shifts towards parasympathetic dominance, preparing you for sleep onset, memory consolidation and glymphatic brain clearance. If you expose yourself to bright, especially blue‑rich artificial light during this window, melatonin is delayed, the circadian phase is pushed later, and sleep architecture becomes more fragmented. From around midnight to 04:00, deep slow‑wave sleep dominates the first half of the night, with major pulses of growth hormone, coordinated tissue repair, immune surveillance and synaptic down‑scaling. Body temperature and blood pressure reach their lowest point roughly between 03:00 and 04:30, while melatonin peaks around 02:00–04:00 in healthy sleepers.

Between about 04:00 and 08:00, the cortisol awakening response builds and peaks around 06:00–08:00, mobilising glucose, raising blood pressure and preparing the cardiovascular and musculoskeletal systems for daytime activity. In this window, clock‑controlled genes in metabolic tissues switch on programmes that prime digestion, insulin sensitivity, and locomotor performance for the active phase. From 08:00 to midday, alertness, reaction time, and executive function typically run higher, and gastrointestinal motility and pancreatic insulin responses are optimised for food intake, making this an ideal window for your largest energy‑providing meal, more complex cognitive work and, where appropriate, targeted movement or rehabilitation.

Between roughly midday and 18:00, coordination, muscle strength and cardiovascular performance often peak in the mid‑afternoon, while inflammatory tone and oxidative metabolism remain adapted to daylight activity. Late‑afternoon bright light exposure and movement further anchor the SCN and peripheral clocks, supporting more stable mood and metabolic flexibility. As the day moves into the 18:00–21:00 window, natural light shifts towards longer‑wavelength reds and oranges, melatonin synthesis gears up, leptin signalling communicates energy sufficiency, and insulin sensitivity declines, meaning the system is no longer well suited to large, dense evening meals. This twilight period is the time to reduce stimulation, dim lights and transition into pre‑sleep rituals that signal safety and predictability to the nervous system.

When we repeatedly push against this natural timing through irregular sleep schedules, late‑night light exposure, back‑loaded nutrition, and chronic stress the amplitude and coordination of these rhythms flatten. Over time, blunted rhythms are linked with increased cardiometabolic risk, mood instability, higher cancer risk and impaired immune function.

Light, electrons, and environmental time‑givers

Light is your dominant zeitgeber (time‑giver), but it is only one of several environmental signals that talk to your clock system. Morning sunlight entering the eyes and hitting the skin triggers the SCN, shuts down melatonin, and initiates a cascade that includes nitric oxide production, the cortisol awakening response, thyroid support, and mitochondrial up‑regulation.

Different parts of the light spectrum carry different biological instructions: UVA and near‑infrared wavelengths help structure water in tissues and liberate electrons, enhancing charge separation across membranes and supporting mitochondrial ATP production and redox resilience. The body is adapted to true darkness at night, punctuated only by low‑intensity, long‑wavelength (fire‑like) light after sunset. In contrast, LED and screen‑based lighting deliver biologically intense blue light at precisely the wrong times, when melanopsin pathways and clock genes are expecting darkness, which suppresses melatonin, elevates evening cortisol and effectively creates a “perpetual daylight” signal.

At a biophysical level, electrons act as the currency of both energy and information flow, moving through respiratory chain complexes, antioxidant systems and signalling proteins. Contact with the Earth’s surface through bare feet on soil, sand, grass, or natural water offers access to a huge reservoir of free electrons that can help buffer oxidative stress and stabilise membrane potentials and redox signalling, especially when combined with full‑spectrum sunlight and structured, mineral‑rich water intake. 

Figure 2 | Light spectrums throughout the day, adapted by R. Jessey.

How our light and tech environment has changed

For most of human history, light exposure was simple, bright, full‑spectrum sunlight by day, moonlight, and firelight by night, with seasonal variation in length of day and light intensity. Our nervous systems and circadian machinery evolved in that context, with sunrise and sunset as non‑negotiable daily signals. In less than two centuries, and especially in the past few decades, we have radically altered that environment. Gas lamps gave way to incandescent bulbs; these were replaced by fluorescent lighting and, more recently, by high‑intensity, blue‑heavy LED lighting in homes, workplaces, and public spaces. On top of that, smartphones, tablets, laptops, and televisions now emit bright light late into the evening, often at arm’s length from our eyes. Many people spend most of the day indoors, under static artificial light, then bathe their retinas in blue light at night while getting almost no true morning sunlight.

At the same time, our relationship with temperature, movement and social rhythms has shifted. Central heating and air conditioning flatten natural temperature cycles, while cars, and desk‑based work reduce the subtle movement cues that used to punctuate the day. Social rhythms, once anchored to daylight and seasonal work, now revolve around 24‑hour access to entertainment, food delivery, and online interaction. We have, in effect, engineered an environment where our biology is constantly being told that it is daytime, even when the sun has set, and where the cues that would once have grounded us in local ecology are muted or absent. It is no surprise that our clocks struggle to stay in tune. Many of us are unable to avoid these situations however there are several things we can do to help mitigate the negative effects.

Figure 3 | Evolution of light over time Dr Alexander Wunsch

Meal timing, metabolism and chrononutrition

Just as light sets the phase of your master clock, feeding rhythms strongly entrain peripheral clocks in metabolic organs such as the liver, pancreas, gut and adipose tissue. Chrononutrition asks how when you eat interacts with what you eat to shape blood sugar control, lipid profiles, body composition, appetite regulation and even responses to medication. Insulin sensitivity and digestive capacity are naturally higher earlier in the day, and the same meal can produce very different glycaemic and lipaemic responses depending purely on timing. Regular late‑night eating is associated with higher levels of adiposity, poorer sleep quality, impaired glucose tolerance and greater inflammatory markers, even when total calories are matched.

Shifting a greater proportion of total energy, protein and fat intake into breakfast and lunch, with a lighter, earlier evening meal, respects circadian variations in insulin sensitivity, digestive hormones, and mitochondrial substrate handling. Clinically, this “front‑loading” of nutrition often improves energy stability, reduces nocturnal reflux and supports easier sleep onset, particularly for people with chronic health conditions. Compressing food intake into a 10–12‑hour daytime window for example, eating between 08:00 and 18:00 without aggressive caloric restriction can improve clock gene expression, cardiometabolic markers and sleep quality, especially when the eating window is anchored earlier rather than drifting into the late evening. 

For chronically ill, underweight, or highly sensitive individuals, this needs careful personalisation and work with an experienced health professional to avoid under‑fuelling, but the core idea remains that consistent, daylight‑aligned eating rhythms are a powerful, gentle way to signal time to the body. Done well, chrononutrition becomes a potent tool to help entrain circadian rhythms, complementing light, movement, and other environmental cues.

Circadian rhythms and your gut microbiome

Your gut microbes also keep time. Across a 24‑hour cycle, different groups of bacteria rise and fall in number and activity, shifting what they are doing for you at different times of day and night. During the active, daylight phase, many microbes focus on helping you digest and metabolise incoming food, producing signalling molecules that talk to your immune system, gut lining and brain; at night, when you are fasting and (ideally) sleeping, microbial activity leans more towards repair, detoxification, maintaining the gut barrier and producing compounds that help regulate inflammation and circadian biology in the host.

These microbial rhythms are closely tied to your own patterns of light exposure, sleep and eating. When you eat at roughly the same times each day, mostly in daylight hours, and protect your sleep at night, your microbiome develops its own strong day–night rhythm that supports blood sugar balance, appetite regulation, immune resilience, and mood.

Practical ways to retrain your circadian rhythms

When I support people with circadian retraining it is about gradually restoring a predictable pattern of light, dark, activity and nourishment that your biology understands. A helpful starting place is to give your body a reliable “bookend” in the morning. Follow my mantra of “daylight before screen light”. Aim to wake at roughly the same time each day, preferably as the sun is rising, open the curtains and window and take in that natural light, where possible, step outside for 5–15 minutes of natural light, even on cloudy days. If getting outside is not possible, sitting by an open window and facing the sky is still useful. Pair this with some gentle movement or a few slow, deep breaths to reinforce “wake‑up” signals. During the day, and especially if you work on screens and artificial lighting all day, schedule in hourly daylight breaks, where you get up to go outside or get to an open window to expose your eyes to natural light.

In the evening, focus on nurturing darkness. When you can, catch every sunset because this sends a circadian signal that gets the body preparing for sleep. Choose a consistent wind‑down time, ideally starting at least an hour or two before you plan to sleep. Dim overhead lights, use lamps with warmer bulbs, reduce screen brightness and, where you can, switch to audio, reading or conversation instead of visually stimulating content. If you are unable to avoid devices, screens, and lighting, consider blue‑light‑reducing settings, overlays, or glasses, but still aim to reduce total exposure. Become strict around your use of devices before bed and set a timer for when you come off of all devices. Please note this cannot apply to those who are using devices to manage health conditions such as insulin / medication pumps etc. Despite this, creating a simple, repeatable pre‑sleep routine helps the nervous system anticipate sleep.

Aligning meals with daylight is another accessible lever. If it fits your situation, work schedule and overall health condition, experiment with having breakfast within 30 minutes to an hour of waking, make breakfast and lunch your most substantial meal and keep your evening meal lighter and finished at least two to three hours before bed. Try to avoid grazing late into the evening, particularly on highly processed, sugary or high‑fat foods that demand more from digestion at a time when your system wants to be shifting into repair mode. For those with limited capacity, even shifting the last meal 30–60 minutes earlier can be a meaningful step.

Where possible, weave in small, regular doses of outdoor time and connection with natural surfaces. A few minutes standing with bare feet on grass, soil or sand, sitting with your back against a tree, tending to a plant pot, or simply opening a window to feel the air and notice the sky all provide gentle anchoring signals. If you spend a lot of time indoors, consider whether you can rearrange your environment so that you sit closer to natural light during the day and have easy ways to reduce light intensity in the evening.

Seasonal cycles, local food, and circadian health

In spring and summer, longer days and increasing light have traditionally been matched with lighter, fresher foods such as tender greens, leafy vegetables, young vegetables and herbs, new potatoes, fresh eggs, fish from coastal waters and animals grazing on rapidly growing pasture. As the season moves into high summer, berries and other sun‑ripened fruits appear, naturally providing structured water, vitamins, minerals, polyphenols, and fibre. These foods, combined with longer daylight and more movement, tend to support detoxification pathways, insulin sensitivity and a more active metabolism, in step with a time of year when our bodies were designed to be outdoors and more physically engaged with the landscape.

As autumn and winter arrive and daylight shortens, traditional diets shift towards heartier, richer foods and include roots, brassicas, nuts and seeds, fattier cuts of meat, bone broths and slow‑cooked dishes that make use of the whole animal. These foods are more energy‑dense and warming, and can support a slower, more restorative seasonal rhythm with longer nights and more indoor time. It is also no coincidence that autumn offers a flush of immune‑supportive foods, deep‑coloured berries, rose hips, apples, elderberries, mushrooms and strong aromatic herbs. These carry vitamin C, polyphenols, antimicrobial, and immune‑modulating compounds just as the body prepares for colder months and a higher burden of winter infections.

Only a few generations ago, autumn was a time for pickling, preserving, fermenting, and curing, stretching the abundance of the growing season into the winter months. Families used the whole of an animal rather than the handful of popular cuts we see today, went fishing in particular seasons, gathered shellfish at low tides and collected wild plants, herbs and fungi from hedgerows, woodland and shorelines. Many of these skills have faded in a remarkably short time. Modern food systems largely ignore seasonal and local cycles, we can buy the same strawberries, tomatoes and salad leaves in December as in June, often grown far away, heavily sprayed, picked unripe and transported long distances. Ultra‑processed foods, available 24/7, add another layer of “timelessness” to eating, with products engineered to taste the same all year round, irrespective of season or place.

This constant, season-less abundance flattens yet another layer of timing cues to the body. When we re‑introduce seasonal, locally appropriate eating even in small steps we give our circadian and metabolic systems a clearer message about where we are in the year and what our bodies might reasonably need. At the same time, it gently brings our attention back to local ecosystems and farming practices, reconnecting our plates to the land, waters and communities that sustain us.

Reconnecting with nature’s rhythms

At its core, circadian retraining is about repairing a broken relationship between daylight and darkness, with the land, with food, with community and with the inbuilt intelligence of our own cells. Re‑aligning with the 24‑hour rhythms invites us to live as part of an ecosystem again, rather than as insulated observers under permanent artificial light, ultra‑processed food, and constant digital stimulation. It is important that we reconnect with the living world that has shaped our biology for millennia, starting with the places and seasons you inhabit. Learning about the seasonal plants and medicines that grow in our hedgerows and woodlands, and in our local climate, helps re‑root us in place. Re‑acquiring traditional skills like foraging, wildcrafting, hunting, fishing, preserving, and simple gardening turns food back into a story of season, soil and sunlight, rather than something created in a lab that comes in a packet.

As humans we have evolved under the reliable signals of natural daytime and night‑time; our modern, artificial world has slowly created a huge disconnect and, alongside it, an epidemic of chronic illness and dysregulated rhythms. Each time we step outside at sunrise, share a seasonal meal, plant a seed, turn down a screen in favour of a conversation, we take a step back towards alignment. In doing so, we are not only supporting our circadian biology but also remembering what it means to inhabit a living world together.

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