hrv.works
Blog
Get the App
Back to Blog
ppgAge•HRV Works Team

Biological Age: The "Speed of Aging" Found in 210,000 Wrists

The sensor that measures your HRV also knows how fast you are aging. The secrets of biological age revealed by data from 210,000 people.

Imagine two 50-year-olds. Both are wearing the exact same Apple Watch. Their ID cards say they are the same age, and the devices on their wrists are identical.

But the time their watches see is flowing differently. One person's body is beating like a 45-year-old's, while the other is aging like a 57-year-old.

The watch already knew the difference.

In 2025, researchers from Apple and Princeton University published a study in Nature Communications that sent shockwaves through the health community. They analyzed data from 213,593 Apple Watch users, covering a staggering 149 million days of data. It was the largest wearable aging study in history.

You check your HRV every morning, right? That green light isn't just measuring your heart rate.

What the Green Light on Your Wrist Knows About You

Have you heard of "Biological Age"? It's different from your Chronological Age.

Chronological age is just a count of how many times the Earth has orbited the Sun since you were born. It's a number we can't control. Biological age, on the other hand, is a report card showing "how functionally old your body is."

What You Already Know: HRV

The HRV we check daily shows "today's condition." It measures how flexibly your autonomic nervous system is reacting.

"I drank last night, and my HRV tanked." "My HRV is low this morning, so I'll take it easy on my workout."

We're used to this, right? It's a check-up on your body's short-term state.

What You Didn't Know: ppgAge

But that same sensor is telling a completely different story. It's reading the age of your blood vessels.

The green light on the back of your Apple Watch—technically called PPG (Photoplethysmography)—shines through your skin to illuminate your blood vessels. Every time your heart beats, your blood vessels expand and contract, changing the amount of light reflected back.

Here's the interesting part: as we age, our blood vessels change. They become stiffer, thicker, and lose elasticity. The shape of the wave (waveform) that reflects back when the heart pumps blood changes subtly with age. Specifically, the waveform around the "diastolic peak" shifts, and deep learning models are incredibly good at spotting this.

The researchers trained an AI on these patterns. The result?

An average error of just 2.4 years.

Without drawing a single drop of blood or running expensive DNA tests, the watch on your wrist estimated biological age. The researchers named this 'ppgAge'.

35 Years Old with 3.5x Heart Disease Risk?

"So my biological age came out a bit higher than my actual age. No big deal, right?"

It's not something to brush off lightly. This is the most chilling part of the study results.

The researchers called the difference between biological age (ppgAge) and actual age the 'AgeGap'.

AgeGap = Biological Age - Actual Age

They tracked people whose AgeGap was +6 years or more—meaning their bodies were biologically 6 years older than their actual age.

ConditionResult
Men aged 35-45, AgeGap +6Heart Disease Diagnosis Rate 3.5x
Women aged 35-45, AgeGap +6Diabetes Diagnosis Rate 2.4x

3.5 times. That's massive. For the 30-40s generation who think, "I'm still young, I'm fine," the watch is sending a clear warning.

What's even more surprising is that the researchers controlled for existing risk factors like smoking, obesity, and family history. The results held true. The watch was seeing "something" that standard hospital tests weren't catching.

The UK Saw the Same Thing

A massive study from the UK Biobank (also using data from 210,000 people) showed similar results.

People whose AI-estimated vascular age was 9+ years older than their actual age had a 2.37 times higher risk of dying from cardiovascular disease. Coronary artery disease, myocardial infarction... almost every cardiovascular disease we fear showed significantly higher risk.

Conversely, those whose biological age was 9+ years younger? All risk markers were lower.

What Makes You Age Faster?

So, the big question: What is driving up my biological age?

The researchers broke down the relationship between lifestyle habits and ppgAge. The culprits are the usual suspects, but seeing the specific numbers makes it hit home.

1. Smoking: The Choice to Fast-Forward 10 Years

The most potent aging accelerator was, unsurprisingly, cigarettes.

Smokers aged 55-65 had a biological age averaging 3.5 years higher than non-smokers.

"Only 3.5 years?" you might ask. That's the trap of averages. If you look at the cellular level, it's much worse.

A study analyzing blood vessel cells from patients undergoing coronary artery surgery found that smokers' cells showed more inflammation, higher oxidative stress, and were essentially "senescent" (old).

The shocking part? Smokers undergoing this surgery were, on average, 10 years younger than non-smokers. They were experiencing the vascular aging others would face 10 years later, all because of smoking.

2. Sleep: Lack of Sleep Ages You

The old saying "Sleep is the best medicine" has been scientifically proven.

In the ppgAge study, every hour of lost Deep Sleep was associated with a 1-year increase in biological age.

There's a study on Centenarians (people living to 100+). Do you know what they have in common?

  1. They maintain Deep Sleep (Slow Wave Sleep) even in old age.
  2. Their sleep-wake schedules are laser-focused and regular.

While you sleep, your body repairs damaged cells and cleans out waste products in the brain. If you cut this time short? You start the next day with repairs left undone. Let that pile up for a day or two, and it becomes aging.

3. Exercise: Use It or Lose It

Looking at men aged 65-75, the group that exercised the least had a biological age 4 years older than the group that exercised the most.

The interesting thing is that this gap widens as you get older. When you're young, you can get by on natural stamina, but as you age, the "speed of aging" between those who exercise and those who don't becomes drastically different.

The Good News: You Can Turn Back the Clock

Feeling depressed thinking, "Is it too late for me?"

Don't be. The most important takeaway from this article starts now. The key feature of biological age is its 'Plasticity'. It can be changed.

The Pregnancy Paradox: Aging, Then Rejuvenating?

A recent study published in Cell Metabolism by Yale School of Medicine researchers is fascinating. They tracked the relationship between pregnancy and biological age.

Pregnancy is a massive stressor on the body, right? Indeed, from early to late pregnancy (about 20 weeks), the biological age of pregnant women increased by about 2 years.

"Pregnancy makes you old? That's sad..."

But here's the twist. Three months after giving birth, their biological age plummeted. It didn't just return to baseline; for some, it became even younger than before pregnancy. Some saw a reduction of up to 8 years in biological age.

Women who breastfed recovered even faster.

The hopeful message this study gives us is this: "Even if stress accelerates aging, if given the chance to recover, our bodies can become young again."

Flipping Genetic Switches with Exercise

What about exercise? A review published in the journal Aging in 2025 provides evidence that exercise turns back the "epigenetic clock."

  • Sedentary middle-aged women who exercised for 8 weeks? Biological age decreased by 2 years.
  • After 6 months of resistance training? The expression patterns of 179 aging-related genes switched back to a youthful state.

It's not just about building muscle. You are literally flipping the DNA switches in your cells back to "youth mode."

HRV: Your Daily Speedometer for Aging

ppgAge isn't explicitly shown in the Apple Watch app as "Your biological age is 45" just yet. (It's likely coming soon—Apple wouldn't sit on data from 210,000 people for nothing.)

But we already have a powerful tool: HRV.

HRV and ppgAge are two sides of the same coin.

  • HRV: "How well recovered is my body today?" (Short-term)
  • ppgAge: "How old is my body overall?" (Long-term)

A meta-analysis of 38,000 people shows that low HRV is strongly linked to increased mortality. Low HRV means the autonomic nervous system has lost its resilience, which is one of the surest signs of aging.

Researchers at Johns Hopkins University found that "for every 10ms increase in HRV, the risk of death decreases by 20%."

What You Can Do Right Now

It's not complicated. The methods to lower ppgAge and raise HRV are surprisingly identical.

  1. Sleep: 7+ hours, consistently. (1 hour of deep sleep = 1 year younger)
  2. Exercise: Cardio that gets you out of breath + Strength training. (8 weeks = 2 years younger)
  3. Breathe: 10 minutes a day of slow breathing to wake up the parasympathetic nervous system.
  4. Quit Smoking: Your blood vessels will start to recover the moment you stop.

Conclusion

You can't change your chronological age. Next year, we'll all be one year older.

But biological age is different. That is in the realm of choice.

The data from 210,000 wrists makes it clear. Even after living the same 50 years, some bodies are worn out like they've lived 60, while others are as fresh as if they've only lived 40.

That difference didn't come from genes, but from daily choices.

Tonight, before you go to sleep, take a look at your Apple Watch. That little machine is reading between the beats of your heart, watching the speed at which you are aging.

When you check your HRV score tomorrow morning, don't just think, "Oh, good condition today?" Think of it as a speedometer showing how well you are braking down the steep hill of aging.

So, what time does your watch say it is?

References

Key ppgAge Research

HRV and Mortality

Pregnancy and Biological Age Reversal

Exercise and Age Reversal

You work hard. But is your body keeping up?

Every morning, see exactly how fast you're aging. Replace vague anxiety with clear peace of mind.

Check My Aging Pace

Written by HRV Works Team

Biological Age: The "Speed of Aging" Found in 210,000 Wrists | HRV Works