Bridgeside Study Explores the Ultradian Rhythm in Humans That “Hasn’t Been Observed Really Ever”

Anyone who has ever flown a red-eye or worked a night shift understands the impact on our bodies from disrupting our circadian rhythm. But there is a lesser-known 12-hour cycle called the 12-hour ultradian rhythm, which has been found to impact both marine animals and mice. After studying and publishing research on the presence of a 12-hour ultradian rhythm in mice, Bokai Zhu, Ph.D., a researcher and Bridgeside resident, began to ask a new question: do humans have the same rhythm?

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In a recent publication in npj Biological Timing and Sleep, Zhu and collaborators, Matthew Steinhauser M.D., and Pouneh Fazeli, M.D, M.PH., at the University of Pittsburgh sought to answer this question. Ultimately, their study found evidence of 12-hour ultradian rhythms in humans similar to those in other animals. According to Zhu, this rhythm in humans “hasn’t been observed really ever” and has potential evolutionary ties.

To confirm the presence of the ultradian rhythm, the white blood cells of three healthy adult men were studied over 48 hours using a 2-hour interval for blood sampling. Beforehand, the subjects reported their usual routines, including sleep schedule and diet, for the study to mimic everyday life. All three subjects presented clear signs of the ultradian rhythm in how their cells handled fundamental processes like mRNA and protein metabolism.

Although the 12-hour ultradian rhythm is known, the reason for the cycle and how it relates to the 24-hour circadian rhythm are still open questions. A few proposed causes include a potential connection to the circadian clock and external factors, the cells having their own 12-hour timer or other harmonic frequencies of the 24-hour period.    

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The shared gene seems present in all species and dates back even further than the sea anemone — suggesting an ancient evolutionary origin. In marine animals, this rhythm was adapted to tide cues; however, why mammals have this gene and how it helps them remains unknown. The researchers suspect the cycle has been co-opted to help deal with metabolic stress that peaks during energy transitions. The peaks are at dawn, when a person hasn’t eaten, and their energy is low, and at dusk, when the person has consumed large amount of food during the day and its surplus energy accumulates, creating stress. Zhu compares the twelve-hour oscillator to an HOV lane. As traffic peaks, the HOV lane opens to increase its capacity for cars. Similarly, the 12-hour interval increases the body’s capacity to transfer information as metabolic stress peaks.

What the 12-hour rhythm is, why it’s important, and why mammals and sea life share it are a few questions Zhu and colleagues hope to research in the future. The lab intends to expand by studying a larger sample size and a wider variety of human subjects, including female, older, or unhealthy individuals.

Read the full publication here.