Long before humans created clocks, calendars, and schedules, our bodies were already keeping time. Earlier this year, while exploring Imbolc and the return of light, I briefly introduced the idea of the circadian clock. In this article, I’d like to slow down and take a closer look at what it is and why it matters.

Every living organism carries internal rhythms that help it respond to the world around it. The rising of the sun, the fading of daylight, changes in temperature, and the passing of seasons have shaped biological systems over millions of years. Deep within our bodies is a network of biological clocks that help coordinate when we are alert, when we rest, and how our energy changes throughout the day.

This internal timing system is known as the circadian clock, which generates the approximately 24-hour cycles we call circadian rhythms. The word circadian comes from the Latin words circa (“around”) and dies (“day”), describing a biological rhythm that follows the cycle of day and night.  While we often associate circadian rhythm with sleep, it influences far more than bedtime. It affects hormones, metabolism, body temperature, immune function, attention, and even emotional regulation.

These rhythms are not unique to humans. Across the natural world, plants and animals have evolved internal clocks that help them anticipate changes in their environment. Plants adjust their growth and photosynthesis according to cycles of light and darkness, while animals use circadian rhythms to guide activity, feeding, migration, and reproduction. Although each species experiences time differently, the underlying principle remains the same: life adapts to the rhythms of the world around it. Circadian clocks across the living world follow a roughly 24-hour cycle, but the rhythm is expressed differently depending on each organism’s relationship with its environment. A plant prepares its photosynthetic machinery before sunrise, a human wakes with morning light, and a bat begins its activity at dusk. 

Although the circadian clock operates within us, it depends on environmental cues to remain synchronised. Natural light is the most powerful of these signals, helping align our internal clock with the 24-hour cycle of our planet. However, modern life often asks us to ignore these rhythms. Artificial lighting, digital screens, irregular schedules, and constant availability can create a disconnect between our biological timing and our daily routines. Understanding the circadian clock is therefore not only a scientific journey into the mechanisms of the body, but also an invitation to reconsider our relationship with time. What happens when we begin working with, rather than against, our internal rhythms?

Our relationship with time extends beyond the daily cycle. As daylight changes throughout the year, our biological rhythms continue to adapt. One study by Grimaldi et al. (2008) found that seasonal changes in light exposure were associated with changes in mood, behaviour, and health-related quality of life. The researchers note that the body’s principal circadian clock, located in the suprachiasmatic nucleus of the hypothalamus, responds to changes in day length. This helps explain why reduced daylight can affect wellbeing for some people, particularly those experiencing Seasonal Affective Disorder (SAD). Alongside circadian rhythms, many organisms also experience circannual rhythms: biological cycles that follow the changing seasons of the year. These rhythms influence processes such as migration, reproduction, hibernation, and seasonal changes in behaviour.

Closing Reflection 🌙

Long before we measured time, our bodies learned to live within it. Perhaps understanding the circadian clock is not about keeping better time, but about remembering the rhythms that have always been there. It reminds us that time is more than something we measure; it is something we experience. Every sunrise and every season offers cues that our bodies have evolved to recognise. By paying attention to these rhythms, we can begin to understand not only how we sleep, but also how we think, feel, and move through the changing year. 

References & Further reading

• Grimaldi, S., Partonen, T., Saarni, S.I. et al. Indoors illumination and seasonal changes in mood and behavior are associated with the health-related quality of life. Health Qual Life Outcomes 6, 56 (2008). https://doi.org/10.1186/1477-7525-6-56
• Press release. NobelPrize.org. Nobel Prize Outreach 2026. Mon. 13 Jul 2026. 🔗Link

Sleep Foundation: What Is Circadian Rhythm?
An introduction to circadian rhythms, how they regulate the body, and why they influence far more than sleep.

National Heart, Lung, and Blood Institute: Your Sleep/Wake Cycle
A clear explanation of the body’s internal clocks, the sleep-wake cycle, and how circadian rhythms affect every organ system.