What Your Apple Watch Is Actually Seeing Before You Flare

I've worn an Apple Watch since the Series 2. Years before I was diagnosed with Crohn's.

After my diagnosis in 2020, I'd notice things. My sleep would fall apart before a flare, which makes sense when pain is waking you up at night. My resting heart rate would drift higher for a few days. I'd feel off before I felt bad. I'd look at the data on my wrist and think something is happening, but I couldn't put it together. I was too busy navigating Crohn's to become a data scientist about it. Figuring out medications, dealing with flares, losing response to Inflectra, switching to Entyvio, getting through a resection, starting Skyrizi. Understanding my Watch data was not at the top of the list.

Then in January 2025, Mount Sinai published a study that connected the dots I couldn't.

The Study

309 IBD patients across 36 states wore Apple Watches, Fitbits, or Oura Rings while answering daily symptom surveys and providing blood and stool samples. The researchers found that physiological signals measured by these devices changed significantly before flares. Not during. Before.

Heart rate variability shifted. Resting heart rate climbed. Daily steps dropped. Oxygenation dipped slightly in Crohn's patients. And these changes showed up as early as seven weeks before symptoms appeared.

Seven weeks. You're still feeling fine. Your Watch already knows something is off.

The study was published in Gastroenterology, which is the top journal in the field. This wasn't a press release from a startup. It was peer-reviewed research from one of the leading IBD centers in the world.

I read it and thought about all the data sitting on my wrist that I never fully understood. I scrolled back through some of my old health data, casually, not scientifically. But the patterns the researchers described matched what I'd been half-noticing for years. That's what made me build Flarity.

What These Signals Actually Mean

Most of us look at our Watch data and think "huh, my heart rate was high today" and move on. But for IBD patients, these metrics are indirect windows into your immune system.

Heart Rate Variability (HRV) measures the variation in time between heartbeats. Higher is generally better. It reflects your autonomic nervous system, the part that handles things you don't think about like digestion, inflammation response, and stress regulation. When inflammation is building in your gut, your nervous system responds. HRV drops. Sometimes days or weeks before you feel anything.

Resting Heart Rate (RHR) is your baseline when you're doing nothing. When your body is fighting inflammation, it works harder. Your resting heart rate creeps up by a few beats per minute. Not enough to notice. Enough for a sensor to detect.

Sleep disruption shows up before flares because inflammation affects sleep architecture. If you have IBD you already know this one. Pain wakes you up. Urgency wakes you up. Even before a full flare, low-grade discomfort chips away at your deep sleep and efficiency. Your Watch tracks this even when you're too exhausted to notice the pattern yourself.

Wrist temperature is a newer metric from Apple Watch. Systemic inflammation can cause subtle temperature shifts. A sustained rise of 0.3 degrees Celsius against your personal baseline is a signal worth paying attention to.

Steps and activity drop before flares for reasons you'd expect. Fatigue, low-grade pain, general malaise. You move less before you realize you're moving less.

None of these signals alone is diagnostic. A bad night of sleep doesn't mean you're flaring. A stressful week at work will tank your HRV too. That's the problem with wearable data in isolation. It's noisy. I know because I spent years looking at my own data and not being able to separate signal from noise.

The Ceiling

Here's what the Mount Sinai study doesn't tell you, and what most articles about it skip: wearable data alone has a hard ceiling.

Your Watch can detect physiological stress. It cannot tell you why. A dropping HRV could be inflammation. It could be a cold. It could be that you stayed up too late. An elevated resting heart rate could be a flare building. It could be caffeine.

And your Watch absolutely cannot assess subjective symptoms. It doesn't know if you're in pain. It doesn't know your bowel frequency. It doesn't know if there's blood. These are the things your GI uses, alongside biomarkers, to determine disease activity.

That was the missing piece I kept running into with my own data. I could see something was happening. I couldn't tell if it was my Crohn's or just life being stressful.

Two Layers, Not One

This is why I built Flarity the way I did.

Flarity monitors the same signals the Mount Sinai study validated: HRV, heart rate, sleep, temperature, steps, SpO2. It tracks them against your personal baseline, not population averages, because what's normal for you is what matters.

When those signals deviate beyond your baseline thresholds, Flarity doesn't panic. It asks you a question. A 30-second check-in. How's your pain? How many bowel movements today? Energy level? That's it.

Your Watch provides the objective layer. Your answers provide the subjective layer. Combined, they produce a risk score on a 0 to 7 scale. This is the same general approach your gastroenterologist uses: biomarkers plus clinical assessment. Labs plus how you're actually feeling.

What It Doesn't Do

Flarity doesn't diagnose flares. It doesn't replace your GI. It doesn't look at your data and say "you are definitely flaring." Inflammation is complicated and individual, and anyone who tells you a wristwatch can make that determination is selling something.

What it does: it watches when you can't. Most of us don't stare at our HRV charts. We don't notice when our resting heart rate drifts up over ten days. We definitely don't connect a bad night of sleep to something brewing in our gut. I wore a Watch for years and couldn't do it manually. The data was there. The pattern recognition wasn't.

Flarity connects those dots and checks in when the dots start to look concerning. When they don't, it stays quiet. No daily surveys. No notification guilt.

What This Means For Your Next Appointment

The Mount Sinai study concluded with a line about "leveraging wearable technology for health monitoring in innovative ways." That's academic speak for: this data is useful, someone should do something with it.

Flarity does something with it. It turns weeks of Watch data into a PDF your GI can read in 10 seconds. Vitals trends, clinical scores, medication adherence, and auto-generated questions based on what your data actually shows.

Your doctor doesn't have time to scroll through your Apple Health app. But a one-page executive summary showing that your HRV has been trending down while your resting heart rate has been trending up? That's a conversation starter.

I'm not going to pretend this is a solved problem. Flare prediction from wearable data is early. The Mount Sinai study is promising but it's one study. Flarity uses a conservative 4-week detection window instead of the 7 weeks the research showed, because I'd rather give you a reliable signal than an early guess.

But the alternative is what we've been doing: waiting until we feel terrible, calling the GI, waiting for an appointment, and trying to remember when it started.

I spent years with data on my wrist that I couldn't interpret. Now it means something.