Years of Fitbit Data Link Steady Daily Rhythms to Slower Biological Aging

The work, published in Nature Communications in 2026 by Shim and Onnela, is an observational study in people. It used data from 2,222 participants in the All of Us Research Program, a large US cohort. Together they contributed 8,447 person-years of data. Observational means the researchers looked for links. They did not change anyone's habits, so the study cannot show that a steadier rhythm causes slower aging.

Biological age here was measured with PhenoAge. It is a score built from nine routine blood markers plus chronological age. The markers include albumin, creatinine, C-reactive protein, glucose and white blood cell count. The score reflects the age at which an average person in a reference group would have the same mortality risk. The study labeled participants as fast agers or slow agers based on their biological aging status. The text we reviewed does not spell out the exact cutoff used to divide the two groups.

The team pulled rest-activity rhythm measures from Fitbit step counts. These captured how strong the daily rhythm was (intensity), when activity peaked (timing), and how regular the pattern was from day to day (stability). Earlier studies often relied on a single week of activity tracking. This one followed people for a mean of 6.84 years.

The headline result: higher rhythm intensity was associated with 26% to 46% lower odds of accelerated aging. In plain terms, people with a more pronounced daily pattern of active days and quiet nights were less likely to be in the fast-aging group.

At the start of the study, 1,021 participants (45.9%) were fast agers and 1,201 (54.1%) were slow agers. The two groups had similar chronological ages, about 61 and 60 years. Their biological ages differed by roughly 12 years: 66.4 versus 54.3.

The activity patterns differed too. Fast agers showed lower peak activity and weaker daytime intensity. They were more active between midnight and 6 a.m., while slow agers kept a more consolidated rest period. Fast agers also took fewer steps on average (7,215 a day versus 8,141) and slept slightly less (6.32 hours versus 6.46 hours). From about age 40 onward, fast agers tended to reach their activity peak later in the day.

Fast agers also had a higher average body mass index, more low income and unemployment, and more lifetime smoking. They had a heavier burden of other health conditions and used more medications. These differences are a reminder that rhythm may be tangled up with other parts of life and health. The paper describes associations, and that kind of tangle is one reason to be cautious.

Sex mattered. Associations of timing and regularity were stronger in females. In males, accelerated aging followed what the authors call a biphasic instability pattern, with early-morning surges and late-evening rebounds.

For you

What does this mean for personal health? The authors suggest that rhythms measured by consumer wearables may serve as digital biomarkers of aging trajectories. That is, a device many people already own might track something linked to how fast the body is aging. The study does not test whether changing sleep and wake times or adding active days slows aging. That remains an open question.

What to watch next

Trials that test whether deliberately steadying daily routines shifts biological age, and whether wearable rhythm scores hold up as a risk measure in other groups.

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