Everyone’s in love with this fancy heart rate monitor—but what does it actually tell you?

As a plugged-in, wellness-savvy person with a propensity for movement (be it running, yoga, or HIIT), you’ve probably experimented with fitness gadgets, watches, and trackers. Over the past few years, tech companies have increasingly been flexing their muscles and showing off innovations that can help people eat better, sleep more soundly, and work out smarter. Because in order to (bio)hack, you first need to track.

So when I started seeing the WHOOP—a “heart rate monitor” that originally cost $500 dollars but is now subscription-based—on the wrists of CrossFit athletes (*waves electronically at my CrossFit-crushes Kelsey Keil and Kari Pearce*) and Olympians (oh hey, Michael Phelps) alike, I was intrigued to say the least.

“The WHOOP tells you the strain of your workout on your body, how well and long you’re sleeping, and how well your body is recovering from your workouts,” says Will Ahmed, Founder and CEO of WHOOP.

What differentiates the WHOOP from just about every other fitness tracker is that it focuses specifically on fitness output and recovery. “When it comes down to it, the WHOOP tells you the strain of your workout on your body, how well and long you’re sleeping, and how well your body is recovering from your workouts,” says Will Ahmed, founder and CEO of WHOOP tells me.“It gives you information about your body that you can truly analyze in order to help you feel better and make better decisions around your wellness.” Cheers to body-knowledge and biohacking.

How does the technology work, exactly?

The always-on wristbands (they just look like a fabric watch bands, BTW) measure heart rate, heart rate variability (HRV), and sleep. “Heart rate variability is the variation in time between each heart beat,” says certified strength and conditioning specialist Rachel Mariotti CSCS. This is different than your heart rate, which measure beats per minute (versus HRV which tells you the variation between the beats).

The WHOOP synthesizes data around HRV, heart-rate, and sleep in order to give users scores on their “Strain” and “Recovery” via its app.

“The variability reflects your ability to adjust between high-intensity, high-demand activity and a resting state, so it’s about your heart beat’s ability to shift throughout the day,” she says. “There are currently no established scientific values of exactly what you want your HRV to be, but a high HRV is ideal and reflects readiness for activity. A low HRV is indicative of increased stress, decreased sleep quality, or too much exercise without proper recovery,” explains Grayson Wickham, DPT, CSCS, founder of Movement Vault, a mobility and movement company.

The WHOOP synthesizes data around HRV and sleep in order to give users scores on their “Strain” and “Recovery” via its app and online dashboard. The users can then use this info determine how hard they *actually* worked out and how well they recovered from their previous sweat-sesh—which Ahmed suggests can help boost performance and prevent injury. Recovery is a vital, non-negotiable part of a any routine (especially a fitness routine that’s performance and goal-oriented like mine), so I ordered one on the spot.

Scroll down to read what I actually got from the luxury-priced wrist-band

As a 9-to-5er trying to balance building an freelance-empire (*hair flip*), becoming a top 100 female CrossFit athlete in my region, and managing a flailing love-life, my recovery was taking a backseat.

I hadn’t made time to visit recovery-focused studios, I was sleeping closer to five hours than eight a night, and it’d been a while since Headspace founder Andy Puddicombe’s dreamy voice entered my earbuds for a guided meditation. “The truth is that athletes and even casual exercisers don’t know what they’re doing to their bodies. They underestimate the importance of sleep, and of recovery in general,” Ahmed told me. And he was right, I felt fine when I fitnessed, but was I recovering? Eh, hard to know. (Or at least it was pre-WHOOP.)

The first three to four days of wear are used to determine a baseline “score” for Strain and Recovery.The app also prompts the user to answer Qs about how they feel immediately following an activity and when they wake up in the morning, so factor into the algorithm and amplify the “I really know you” vibe of the device. Like oh, you want to know how stressed I am? You want to know how if I fell asleep with a book in my hands? And there are easy-to-swipe-through interfaces for sleep, strain, and recovery with readouts and graphs that track progress by the day, week, and month.

Truthfully, it was stunning how closely the data matched what I felt. When I felt crappy, it told me my recovery was crappy. When I felt like Beyonce, it told me my recovery was one hundred.

After the initial days of set-up, the WHOOP told me how much sleep I needed to wake up 100 percent recovered, how much I’d recovered when I woke up based on the duration and quality of my sleep, and when a workout might have been too hard.

Truthfully, it was stunning how closely the data matched what I felt. When I felt crappy, it told me my recovery was crappy. When I felt like Beyonce, it told me my recovery was one hundred. On the days I did doubles, my sleep reflected that I’d slayed in the gym. The biggest disparity between what I *thought* and reality was my sleep. Almost every night, I was getting 30 to 60 minutes less of sleep a night than I thought, thanks to mid-snooze wake-ups that I either didn’t remember or didn’t factor into my sleep math.

In order to *really* use the feedback from the WHOOP, I’d need to shift my priorities and make more time for optimal recovery.

Something else I noticed, is the three days in the month when I had my period, the quality of my sleep plummeted, even when my hours spent in bed were the same. Ahmed also suggested that people noticed a difference when they meditate, and while I didn’t meditate, my recovery was better on the days I did some yoga or RomWOD than the days I did not. I don’t drink alcohol, but Ahmed also mentioned that “it’s common for users to begin to drink less alcohol because they start to realize the impact that it has on their sleep and recovery.”

After a month using the WHOOP and poring through the data, my verdict is that in order to *really* use the feedback on the app I’d need to shift my priorities (AKA stop trying to do it all) for optimal recovery. Will I ultimately, work out less to reduce strain or spend less time in my emails before bed to get more sleep? I’m not sure. On one hand, I think the work/workout hustle is unavoidable at this moment. But on other, I’m glad I know how much this go-hard schedule is affecting my body from a heart-deep level. Because as it turns out: Rest isn’t just a luxury; now, it’s an investment.

If you’re looking for more fitness apps to add to your phone, check out Obé—plus, this list of the top workout ones of 2018, so far. Or go all-in, and biohack your DNA and slow aging.