Worn-Out Body Clocks: Weak Light and Temperature Rhythms Track With Obesity and Metabolic Risk

The human body runs on a set of internal clocks that are synchronized, above all, by light. When those clocks fall out of step with the day-night cycle, a growing body of evidence suggests, the metabolic machinery that governs blood sugar, fat storage and appetite begins to misfire. Now one of the largest real-life studies ever conducted on this question has added a striking new layer: the strength of a person’s daily rhythms in light exposure and wrist temperature, measured continuously for a full week with wearable sensors, is linked to triglycerides, insulin resistance, body fat and obesity itself. The findings, published in the International Journal of Obesity, come from the ONTIME-MT study and suggest that the humble smartwatch or actigraph on your wrist may one day help flag metabolic trouble long before a diagnosis is made.

The research team, led by investigators at the University of Murcia in Spain together with collaborators at Harvard Medical School, Massachusetts General Hospital and the University of Colorado Boulder, recruited 809 healthy adults of European descent from the Spanish ONTIME-MT cohort, a study registered with ClinicalTrials.gov under number NCT03036592. The participants, whose average age was about 37 years and roughly 71 percent of whom were women, wore sensors for seven consecutive days while going about their ordinary lives. Light exposure was recorded continuously, and so was wrist temperature, a surprisingly informative physiological signal that rises during rest and sleep as blood flows back toward the skin and falls during activity and wakefulness. Rather than examining each rhythm metric in isolation, the researchers used principal component analysis, a statistical technique that compresses many correlated variables into a small number of composite factors, to capture the overall architecture of each person’s circadian functioning.

The analysis produced three factors in each of the two domains, light exposure and wrist temperature, and together these factors explained roughly 73 percent of the variance in light rhythms and 80 percent of the variance in temperature rhythms. In both domains, the first factor carried the greatest weight, accounting for about 41 to 42 percent of the variance. This dominant factor reflected what the researchers call rhythm robustness: a combination of high relative amplitude, meaning a large difference between the peaks and troughs of the daily cycle, and greater interdaily stability, meaning the rhythm repeats reliably from one day to the next, along with the person’s average levels of light exposure or wrist temperature. In practical terms, a person scoring high on this factor lived a life of bright days, dark nights and a body temperature rhythm that rose and fell like clockwork, day after day.

What happened when those robustness scores were matched against blood tests and body measurements was the heart of the study. People with stronger, more stable light exposure rhythms had lower serum triglyceride concentrations, a relationship that reached statistical significance at P equals 0.013. Triglycerides are fats circulating in the blood, and elevated levels are a well-established marker of cardiovascular risk. The association between light and blood fats is biologically plausible: light is the primary external synchronizer of the circadian system, and laboratory work has shown that when the circadian clock is misaligned with behavioral cycles, glucose tolerance, insulin sensitivity and lipid metabolism all deteriorate. Animal studies have gone further, showing that light at night can increase body mass partly by shifting the timing of food intake into hours when the metabolism is poorly prepared for it.

The wrist temperature results were, if anything, even more compelling. Higher wrist temperature Factor 1, the robustness composite, was associated with lower body mass index and lower body fat percentage in women, as well as lower fasting insulin, lower HOMA-IR, a standard index of insulin resistance calculated from glucose and insulin levels, lower triglycerides and higher HDL cholesterol, the so-called good cholesterol. All of these associations were statistically significant at P values below 0.05. Wrist temperature is an integrated readout of the circadian system because it reflects the interplay between the autonomic nervous system, vascular tone and the master clock in the hypothalamus. Previous work by some of the same investigators had shown that obese women display flatter, less rhythmic wrist temperature profiles than normal-weight women, and the new study extends that observation to a much larger sample and ties it directly to a panel of metabolic risk markers.

Perhaps the most intriguing finding came from the mediation analyses, which probed whether the two rhythm domains influence each other in a way that shapes metabolic outcomes. The results suggested that the interdaily stability of wrist temperature rhythms may act as a conduit in the relationship between light exposure stability and triglyceride levels, accounting for approximately 11 percent of the total effect, with a P value of 0.021. In other words, part of the reason that well-structured light exposure is associated with healthier blood fats may be that stable light patterns help stabilize the body’s internal temperature rhythm, which in turn supports healthier lipid metabolism. This kind of chained relationship, from environmental signal to physiological rhythm to metabolic outcome, is exactly what circadian theory would predict, but demonstrating it in free-living humans with real-world sensor data is rare.

The study’s design carries important caveats. It was cross-sectional, meaning that light, temperature and metabolic markers were all measured at roughly the same time, so the direction of causality cannot be established with certainty. It is entirely possible that people with healthier metabolisms simply live more regular lives, or that obesity itself flattens circadian rhythms rather than the other way around. The cohort consisted of healthy adults of European descent from a single Spanish population, which limits generalizability to other ethnic groups and to people with existing metabolic disease. The authors are careful on this point: the findings show consistent associations, not proof that stabilizing your rhythms will lower your triglycerides. Still, the consistency of the pattern across two independent physiological domains, and the dose-response-like structure revealed by the principal component analysis, strengthens the case that rhythm robustness is genuinely informative rather than statistical noise.

The broader context makes these results timely. A landmark 2024 analysis of more than 88,000 people wearing light sensors found that brighter nights and darker days predicted higher mortality risk, and a growing literature links circadian disruption, from shift work to evening chronotypes to social jet lag, with cardiometabolic disease. Controlled laboratory experiments by Frank Scheer and colleagues, a co-author on the new paper, have shown that forced circadian misalignment in humans raises glucose and insulin levels and blood pressure within days. What the ONTIME-MT study adds is a demonstration that these relationships are visible in ordinary people, in their own homes, using nothing more than a week of passive wrist-worn monitoring and a well-chosen statistical framework.

The practical implications are twofold. First, the study highlights the potential of circadian-related markers derived from wearable technologies as non-invasive indicators of metabolic risk. Instead of relying solely on blood draws taken at a single moment, clinicians of the future might read a week of light and temperature actigraphy to gauge how robustly a patient’s internal clocks are running, and use that information to identify people who would benefit from lifestyle interventions. Second, it points toward concrete behavioral targets: brighter light during the day, darker environments at night, and daily routines regular enough to keep the body’s temperature rhythm oscillating with a strong, reliable beat. None of this replaces diet and exercise, but it suggests a third pillar of metabolic health that has been hiding in plain sight, glowing on our screens and pulsing quietly at our wrists. As wearable sensors become ubiquitous, the science of chronodisruption is moving from the laboratory bench to the population scale, and the body’s daily rhythms are emerging as a vital sign in their own right.

Subject of Research: Associations between daily light exposure and wrist temperature circadian rhythms and metabolic risk markers in healthy adults

Article Title: Disrupted daily rhythms in light exposure and wrist temperature are associated with metabolic risk markers and obesity

Article References: Dozzani, I., Rodríguez-Martín, M., Dashti, H. S., Vetter, C., Fernández-Breis, J. T., Florez, J. C., Saxena, R., Scheer, F. A. J. L., & Garaulet, M. (2026). Disrupted daily rhythms in light exposure and wrist temperature are associated with metabolic risk markers and obesity. International Journal of Obesity. https://doi.org/10.1038/s41366-026-02225-w

Image Credits: AI Generated

DOI: 10.1038/s41366-026-02225-w

Keywords: circadian rhythms, light exposure, wrist temperature, obesity, metabolic risk, triglycerides, insulin resistance, wearables, chronobiology, HOMA-IR, interdaily stability, ONTIME-MT