Does everyone have Circadian Rhythms?

By the Be Awake Aware Alive team

If you've ever wondered why you feel alert at certain times of day and drowsy at others, you've experienced your circadian rhythm in action. But does everyone have one? The short answer is yes — but as with most things in biology, the full story is a bit more nuanced.

What Exactly Is a Circadian Rhythm?

A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. The word comes from the Latin circa (meaning "around") and dies (meaning "day"). These rhythms aren't just about sleep — they influence hormone release, body temperature, appetite, digestion, and even heart rate.

In humans, nearly every tissue and organ has its own circadian rhythm, and collectively they are tuned to the daily cycle of day and night. This internal timekeeping system is driven by a "master clock" — a cluster of nerve cells in the brain called the suprachiasmatic nucleus (SCN).

The Universal Nature of Circadian Rhythms

Circadian rhythms aren't unique to humans. In fact, they appear to be a fundamental property of nearly all life on Earth. From cyanobacteria to fungi, plants, and animals, organisms across every domain of life — Bacteria, Archaea, and Eukaryota — display circadian rhythms.

Research suggests that the last universal common ancestor (LUCA) and all subsequent life forms evolved in response to the Earth's daily cycle of day and night. Even single-celled algae that lack an internal clock can't survive. In essence, circadian rhythms are an "overall feature of life" that has been conserved throughout evolution.

Scientists have even identified a universal marker for circadian rhythms across all life forms: oxidation-reduction cycles of peroxiredoxin proteins. This suggests that the ability to keep time on a daily cycle is deeply embedded in the biology of living things.

So Does Every Human Have One?

Yes — every human being has a circadian rhythm. It's built into our biology at the genetic and cellular level. Your body's internal clock runs even in the absence of external time cues, a phenomenon known as "free-running".

However, there's an important catch: not everyone's circadian rhythm is synchronized to a 24-hour day.

The Blind and Non-24-Hour Disorder

For sighted people, light is the most powerful signal that resets the internal clock each day. But for individuals who are completely blind and cannot perceive light, this synchronization can break down.

About 50% of totally blind people develop a condition called Non-24-Hour Sleep-Wake Disorder (N24HSWD). In these individuals, the internal clock runs on its own cycle — typically slightly longer than 24 hours — and gradually drifts out of sync with the external day-night cycle.

This isn't because they lack a circadian rhythm — they absolutely have one. It's just that their rhythm isn't "entrained" (synchronized) to the 24-hour solar day. Their internal clock free-runs, leading to cycles of insomnia and daytime sleepiness that shift progressively later each day.

Irregular Sleep-Wake Rhythm Disorder

There is also a rare condition called Irregular Sleep-Wake Rhythm Disorder (ISWRD), characterized by the relative absence of a clear circadian pattern in the sleep-wake cycle. People with this condition may sleep in multiple short naps throughout the day and night rather than in one consolidated block.

Even here, though, the underlying circadian mechanism isn't absent — it's simply disorganized or poorly expressed. The biological clock is still present, but its output is fragmented.

Chronotypes: Morning Larks vs. Night Owls

It's also worth noting that while everyone has a circadian rhythm, not everyone's rhythm is the same. Humans fall into different "chronotypes" — some people are natural early risers ("larks"), while others function best late at night ("owls"). These differences are innate and can vary significantly from person to person.

This variation is normal and doesn't mean someone lacks a circadian rhythm — it just means their internal clock is set to a different phase.

The Bottom Line

Yes, everyone has circadian rhythms. They are an ancient, universal feature of life, deeply embedded in our biology from the cellular level up. While some people's rhythms may be out of sync with the external world — due to blindness, certain disorders, or simply being a night owl — the internal clock itself is always there, ticking away.

Your circadian rhythm is as fundamental to your biology as your heartbeat. It's not a question of if you have one, but rather how well your internal clock is aligned with the world around you — and what you can do to keep it healthy.

With thanks to KoolShooters at pexels.com for the great image