Human Body’s Hidden Clock: The Power of 12 Hour Circadian Rhythms

A recent review in npj Biological Timing and Sleep sheds light on a lesser-known body rhythm: the 12 hour cycle. While most of us are familiar with the 24 hour circadian clock that affects sleep and energy, scientists are now discovering that many of our genes also follow a 12 hour rhythm. These patterns may be just as important for our health, regulating metabolism, stress, and immune function. ⁽¹⁾

What Are 12 Hour Rhythms?

Surprisingly, some of your genes switch on twice a day, not just once. These patterns are called ultradian or circasemidian rhythms. Recent studies show that they aren’t just found in sea animals that live by the tides but also exist in mice and humans.

Some scientists think these rhythms may be left over from ancient tidal clocks used by marine creatures. Others believe they’ve evolved to help us handle daily stress, manage meal times, and keep our body systems in balance.

How Do They Compare to Our Daily Body Clock?

The human body’s main clock, the circadian rhythm, runs on a 24 hour cycle. This system controls sleep, hormones, and alertness and relies on proteins like CLOCK, Bmal1, PER, and CRY.

But in sea animals, there’s another clock: the circatidal rhythm, which follows the tides every 12.4 hours. These rhythms help creatures survive changing sea conditions. Interestingly, some of these animals still show strong 12hour rhythms even when their circadian clock genes are turned off. This suggests 12 hour cycles may be powered by a different system altogether. ⁽²⁾

Mice and 12 Hour Rhythms

In mice, scientists discovered a 12 hour rhythm in the liver, where many important genes are active. What’s amazing is that these rhythms continue even in total darkness or in isolated cells, meaning they don’t rely on light or the brain.

A protein called XBP1, which helps cells manage stress, plays a key role. But even when this protein is removed, some 12 hour cycles remain. This tells us that more than one system is likely involved, including factors like feeding times and stress levels.

Do Humans Have These Rhythms Too?

Yes! In a small study, researchers tracked gene activity in three people over two days and found over 600 genes that followed a clear 12 hour pattern. These genes were involved in metabolism, immune health, and stress responses, much like in mice.

Like in mice, human 12 hour rhythms appear independent of light cues, suggesting these cycles are hardwired and possibly governed by metabolic or stress related triggers. The timing of these cycles was slightly different for each person, likely due to differences in lifestyle, meals, and sleep habits. Still, the similarity to mouse data suggests these rhythms are real and biologically important.

Dr. Arpit Bansal, General Surgeon, in his post on Instagram talked about the circadian rhythm:

Can Disrupted Rhythms Affect Health?

Yes. Changes in 12 hour rhythms have been linked to health problems. For example, in people with schizophrenia, brain tissue showed disrupted 12-hour gene patterns, especially in areas related to brain function and protein maintenance.

In mice, obesity and irregular eating can throw off these rhythms. Irregular eating patterns, chronic stress, and sleep deprivation may throw off these 12 hour rhythms, potentially impairing digestion, immunity, and even mood regulation. That means keeping these patterns stable might protect against metabolic issues, mental health disorders, and more.

Just as circadian medicine has helped treat sleep and hormone problems, understanding 12 hour cycles could lead to new ways to manage diseases.⁽³⁾

The Bottom Line

Twelve hour rhythms are an important layer of the body’s internal clock, helping to regulate key systems like energy use, immune response, and stress management. They might be older than we think, possibly dating back to our ocean-dwelling ancestors. Disruptions in these rhythms are linked to conditions like obesity and schizophrenia, so learning more about them could open new doors for personalized healthcare and disease prevention.

References:

1. Emery, P., Gachon, F. “Biological rhythms: Living your life, one half-day at a time”. npj Biol Timing Sleep, 2, 21 (2025). https://doi.org/10.1038/s44323-025-00037-1

2. Hafker, N. S. et al. “Rhythms and Clocks in Marine Organisms”. Ann. Rev. Mar. Sci. 15, 509–538 (2023). https://www.annualreviews.org/content/journals/10.1146/annurev-marine-030422-113038

3. Vijay Kumar Malesu, “Experts review how 12-hour biological cycles operate in mice, with clues in humans”, News Medical Lifesciences, Last modified on June 4, 2025.

   https://www.news-medical.net/news/20250604/Experts-review-how-12-hour-biological-cycles-operate-in-mice-with-clues-in-humans.aspx#:~:text=The%20discovery%20of%2012-hour,than%20light%20or%20brain%20signals

(Rehash/Dr. Hansini Bhaskaran/MSM)