Whoop describes itself as a “human performance company,” and its primary goal isn’t to show you your emails or the time. It has one job, and that job is to show you how to perfectly balance your energy levels, rest quality, and effort based on your specific lifestyle, health level, and body. That said, I kept glancing down at my wrist for some kind of feedback and then remembered to grab my phone and go into the app to check my scores. This is no doubt because I’m used to using an Apple Watch, and I have to admit that I do like a tracker with a watch face—it felt a little strange wearing something that was just a band.

The Apple Watch is a great companion, encouraging me to hit my step, stand, and sleep goals, and I’ve always appreciated the simplicity of the goals it sets – close the rings, gold star for you. Whoop’s insights, however, are a little more granular. Photodiodes on the underside of the band stay pressed against your skin, tracking your heart rate and breathing patterns to create intelligent algorithms that then provide a daily score for sleep, exertion, and recovery.

After a few days of tracking, the app starts showing you graphs and goals based on your behavior and other body data. Within a few days, I could see how much energy I was using versus how much I should be using based on how much rest I was actually getting, how much sleep debt I had, and how hard I should be pushing my body the next day. It was all so in-depth and felt like I had a coach with me 24/7.

I learned that I should actually be going to bed much earlier – something I still don’t fully believe – and that in order to feel fully rested for my level of activity and exertion, I should be getting a few extra hours of rest each night. Some days I was able to find the perfect balance of rest and exertion, but other days the app told me to take it easier to avoid exhaustion.

What I liked most about the strain monitor was the variety of exercises that were included in the app. You can choose everything from Barry’s and interval training to yoga and cycling for a more accurate measurement of how you’ve moved your body, and also select options like walking, carrying a toddler and doing housework. My graphs showed me how much I needed to exercise and rest for optimal performance, and how close I was to reaching my goals with a percentage score, which made the insights much easier to follow.

Of course, the app also showed the classic markers that many other fitness apps offer, like calories burned and steps taken, but there was so much more to the data. I can totally see why biohacking nerds prefer the Whoop, and why it’s easy to get a little obsessed with your insights.

Of course, the more people who use Whoop, the more people contribute to the health data the app collects, which is also useful for establishing a baseline for different body types and lifestyles. My app told me that for someone my size and build, with my specific lifestyle, I was achieving about 84 percent of my goals, compared to other people like me who used the app. This insight is great if you enjoy a bit of healthy competition, of course, but there are other benefits to this level of collected data.

Pregnant women using the app have contributed to new research and data about women’s health that we’ve never had before, and Ahmed said he’s now working with the app’s data science team to optimize the insights Whoop can provide so that the band can make more intelligent suggestions. If Whoop can tell you when to rest and when to exercise during your pregnancy, that’s great, but if it can predict your exact due date, that could be incredibly useful for the later stages of your pregnancy, too.

I found myself checking the app far more often than I normally would on my Apple Watch or Oura app, and felt a little guilty when I wasn’t getting enough rest. The irony of this kind of competitive wellness is that I was actually being pressured to get more sleep and be kinder to myself – things you might not normally associate with a community of incredibly driven and puritanical fitness fanatics. But that’s wellness for you. Over the years, it’s become a much more accessible and welcoming space, and I found myself in groups with women who were clearly professional athletes and people who seemed to be just starting out on their health journeys.

What makes Whoop 4.0 different?

The brand says the 4.0 is 33 percent smaller than its predecessor, with improved sensors so it can detect your heart rate much more accurately. As far as fitness trackers go, I’m not a fan of wearing anything bulky or heavy on my wrist, and the device felt lightweight with a softer band that didn’t feel uncomfortable or weigh me down.

The smaller size is a bit more comfortable, but there’s another reason it’s slimmed down: the launch of the brand’s new clothing line, Whoop Body. The line features pockets specifically made for the new unit, meaning you can now wear your 4.0 somewhere other than your wrist. This is probably a good thing, as many sports scientists say the wrist isn’t the best place to measure heart rate. Now, you can use the range to store your 4.0 on your torso, waist, or calf.

Or, if you want to take the advice of Olympic gold medalist Tom Daley and fancy something a little different: “You can just put the 4.0 device in your underwear and you’ll get the same data and information.”

The charging pod is also much smaller and waterproof to 1m – so it’s no big deal if you accidentally jump in the shower with it on. However, I did forget to take mine off once and because the band is knitted it’s not waterproof so it felt a bit squishy and uncomfortable on my wrist afterwards. After that experience I opted to take the band off when showering and when I went for a dip in the pool on a spa weekend.

Even with its smaller size, the 4.0 still has the same five-day battery life as its predecessor, the 3.0. I had to charge it twice a week, so the claim was accurate. The 4.0 also has an alarm, which can wake you with a gentle vibration either at a time of your choosing or after the app recommends the amount of sleep you need that day, depending on your lifestyle. I set fixed timers for the week and let the app calculate my wake-up time on the weekend.

The 4.0 also measures skin temperature and blood oxygen saturation. Low blood oxygen levels can potentially indicate health problems – see dehydration if you’re exercising or training hard during this mini heatwave.

Does it look good?

The new superknit straps (£44, Whoop.com ) feel a lot softer than the previous ones, and they come in a variety of colours, with different coloured hardware. Black seems to be the default among most users, but personally? I love being able to match the colour of the strap and buckle to my outfit. It’s a little touch that ups the game, and is why I opted for a gold Apple Watch and gold Oura Ring . Sure, black is sporty, but I like having dressier options too.

The 4.0 also has a new slider system, making it easier than ever to swap your 4.0 between different straps. There are plenty to choose from, so if you’re worried about your Whoop clashing with your outfit, you don’t have to choose between a certain look and an interruption in your data. I tried the black strap for a week, then swapped it out for the stone-colored superknit strap, which suited my personal style much better – I love my neutrals.

How much does it cost?

For existing users, the new unit is free. You can easily upgrade via your app and just wait for it to arrive in the mail.

If you’re new to Whoop, there’s one thing to keep in mind. You don’t buy Whoop; you join. The brand’s subscription model requires users to sign up for at least six months at £27 per month – the price drops the longer you subscribe. A 24-month subscription is the best value. Once you sign up, you get the band for free.

It’s a financial commitment, but with more features and data than ever before, it could be worth it for you based on your health, sleep, and wellness goals. The app acts as your personal coach, there are community groups you can join where you can connect with other Whoop members, and you have access to videos that explain every health metric, as well as different workout styles, rest tips, and sleep advice.

If you’d like to give Whoop Body a try, the garments range from £49 to £94.