Fast Agers Took 900 Fewer Steps a Day in This 7-Year Fitbit Study

Your fitness tracker counts your steps. But it also records something more subtle: the shape of your day.

When you get moving, how active you are at your peak, how quiet your nights are, and how regular it all is.

A new study from Harvard looked at years of that data from 2,222 Fitbit users. It compared their daily rhythms with blood tests that estimate biological age.

People with a stronger, more regular daily rhythm were less likely to be ageing faster than their years. The study can’t prove the rhythm is the cause, but the pattern was clear.

What is biological age?

Your chronological age is simply how many years you’ve been alive. Your biological age is an estimate of how old your body seems, based on health markers.

Two people born in the same year can age at different speeds. One may have the blood markers of someone younger, the other of someone older.

This study used a measure called PhenoAge. It combines nine routine blood test results with your actual age.

The nine include markers of kidney and liver function, blood sugar, inflammation and immune cells. Together they estimate age-related risk.

The authors note that PhenoAge predicts disease, physical function and death. They also stress it is a clinical measure, not a molecular “biological clock”.

In the study, anyone whose PhenoAge was higher than expected for their age was classed as a “fast ager”.

The study: years of Fitbit data

Who took part

The research was carried out by two researchers at Harvard’s school of public health.

It was published in Nature Communications in August 2026.

They used the All of Us Research Program, a huge US health study funded by the National Institutes of Health.

Some participants share data from their own Fitbit devices.

The team found 2,222 adults with at least a full year of step data. Each also had at least two sets of blood tests in their health records.

What the researchers measured

From the step data, they described each person’s 24-hour activity rhythm. That included:

• Intensity: how strong the difference is between active days and quiet nights.
• Timing: when in the day activity peaks.
• Regularity: how similar the pattern is from one day to the next.

They then tracked how these rhythms related to PhenoAge over the years.

What the study found

A stronger rhythm went with slower ageing

The clearest result was about intensity. People with a stronger daily rhythm, active in the day and properly at rest at night, were less likely to be fast agers.

Higher rhythm intensity was linked to 26% to 46% lower odds of accelerated ageing, depending on the measure.

These are odds ratios, which show relative differences. The paper doesn’t give absolute risks.

Timing and regularity mattered too

Later peak activity was linked to a higher chance of faster ageing. More regular rhythms were linked to 9% to 13% lower odds.

The fast agers also took fewer steps on average: 7,215 a day, compared with 8,141 for slow agers.

Men and women differed

The link with intensity held for both sexes. But timing and regularity mattered more for women.

In women, later activity and later rest onset were linked to 12% to 18% higher odds of faster ageing. Greater regularity was linked to 15% lower odds.

In men, the researchers found a different pattern. Fast-ageing men often had a burst of activity early in the morning and another rebound late in the evening.

The study had fewer men than women, which made the male findings less certain.

Why the result needs caution

It can’t show cause and effect

This was an observational study. It found links between rhythms and biological age, but can’t show the rhythm caused the difference.

The authors say reverse causality “cannot be excluded”. Poorer health could disrupt someone’s daily rhythm, rather than the other way round.

A healthy, active group

The volunteers were not typical. Most were women, white and college-educated, and they brought their own Fitbits.

The authors note the middle-aged and older participants were more physically active than the national average.

What the tracker misses

Fitbit step counts can miss cycling and strength training. They also can’t always tell when someone has taken the device off.

PhenoAge can shift with short-term changes in blood results, too. So it’s a useful marker, not a perfect one.

What earlier research found

A large UK study

A 2023 study in The Lancet Healthy Longevity followed 92,614 UK Biobank participants who wore a wrist accelerometer for a week.

People with a weak day-night activity rhythm had a 54% higher risk of dying during about six years of follow-up. They were also more likely to develop heart, lung and infectious diseases.

Diabetes risk

A 2025 UK Biobank study of 74,165 people found that a weak rhythm was linked to a 48% higher risk of type 2 diabetes. A later peak in activity was linked to a 25% higher risk.

How it fits together

Earlier studies mostly relied on a single week of wear. The new study adds years of everyday data and repeated blood tests.

It strengthens the case that your daily rhythm says something about your health. It doesn’t yet show that changing the rhythm changes how fast you age.

How much should you trust this?

Early. It’s a large study with years of real-world data. But it’s observational, and the group was healthier than most.

What makes it convincing

• Years of continuous wearable data, not just a week’s snapshot.
• Repeated blood tests let the researchers track change over time.
• Results held up across many sensitivity analyses, including adjustments for exercise and sleep regularity.
• The study had no funding, and the authors declared no competing interests.

What makes me cautious

• It can’t rule out that poor health causes weaker rhythms.
• The volunteers were mostly white, female, educated and active.
• Step counts miss some kinds of exercise.
• PhenoAge is an estimate, and it can move with short-term changes.
• The paper reports odds, not absolute risks.

What this means for you

• Be active in the day. The strongest link was with daytime activity. The NHS recommends adults do at least “150 minutes of moderate intensity activity a week”.
• Rest properly at night. A clear contrast between active days and quiet nights was part of a strong rhythm.
• Keep a regular routine. Similar timing from day to day was linked to slower ageing, especially in women.
• Don’t panic over your tracker. No app can tell you your PhenoAge from your steps alone.
• Talk to a doctor about health worries. Biological age tests are not a diagnosis.

The NHS physical activity guidelines for adults are a simple, practical starting point.

In this short video from Wellcome, you can see how your body clock shapes your day:

What we still don’t know

1. Does changing your rhythm slow ageing? That needs an intervention trial.
2. Does it hold in less healthy groups? The authors call for more diverse studies.
3. Why do men and women differ? Hormones and behaviour may play a part, but it’s unproven.
4. Would better data change the picture? Raw accelerometer data could capture cycling and strength training.
5. Which matters most? Intensity, timing and regularity overlap, and their separate effects are unclear.

My take: the shape of your day matters

What I like about this study is that it turns everyday data into something meaningful. Years of ordinary Fitbit records tell a story that a week of wear can’t.

I’m less convinced by the idea, already popular online, that your watch can tell your biological age. This study doesn’t show that.

But the message is sensible and cheap. Move during the day, rest at night, and keep it fairly regular.

Your body clock likely benefits, even if the ageing link isn’t proven yet.