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In bed with the Apple Watch Series 8 and its new temperature sensor

Just don’t forget to charge it before you turn off the lights.

The Apple Watch Series 8. (Carolyn Fong for The Washington Post)

When I’m running — which doesn’t happen as fast or as frequently as it should — there’s always a few minutes where my heart feels like it’s going to beat right out of my chest.

It's a strangely comforting sensation to me, at least until it started happening while I was sitting still.

Three years ago, my resting heart rate spiked to around 120 beats per minute while I just sitting around reading. And if I’m honest, it was mostly the readings from my Apple Watch’s electrocardiogram feature that got me — a very reluctant patient — to go to a hospital and confront the issue.

Ever since that day, I’ve tried out all sorts of wearables to see if they can help me live a more intentionally healthy life. And last month, I strapped on the Apple Watch Series 8 to see how the company’s wearables have changed since the day one of them forced me to rethink my relationship with my body.

Here’s what you should know:

More ways to track your vital signs

Practically speaking, the Series 8 isn’t all that different from last year’s Apple Watch, or even the one before that. The biggest changes here are the new sensors Apple packed in here, like a system for sensing your temperature, and a specialized motion sensor tuned to detect car crashes.

The second sensor is something I hope you’ll never need to use and can’t really test short of actually crashing a car. (For better or worse, slamming the device onto a hard, carpeted floor wasn’t realistic enough.) But temperature is a different story.

When you look at other observations alongside them, subtle shifts in temperature can sometimes help signal bodily change. For instance, according to the Oura ring — my wearable of choice for vital sign tracking — my temperature was noticeably warmer in the two days before I tested positive for covid. In hindsight, those numbers plus the subtle fatigue that set in were a sign.

Wearable tech can spot coronavirus symptoms before you even realize you’re sick

There are actually two temperature sensors in the Series 8: one built into the outer face of the watch, and another in the domed bit that presses into your wrist. But here’s the rub: you can’t use them on-demand the way you would a thermometer. (In other words, don’t buy one in hopes of sussing out the early stages of a fever.)

Instead, those sensors go to work while you sleep in a process that leaves you not with raw temperature numbers, but the variation from your baseline temperature. That baseline, by the way, takes five nights to calculate and remains completely invisible to you.

So far, I’ve been pleased with how accurate the watch’s temperature sensing skills have been. My nightly results usually differ by a few fractions of a degree compared to the Oura ring, which makes sense — it’s not unusual for my wrist to be covered by sheets while my fingers loll in the breeze.

With few exceptions, though, both wearables typically agree on direction: that is, they're both pretty good at telling when I'm sleeping a little warmer or cooler than usual. But is any of that data actually helpful?

It's a bit early to say, honestly. Apple says this temperature tracking can be helpful for ovulation estimates and family planning, but we're not convinced that a couple weeks is enough time to really get a feel for this feature in action — we'll revisit this once we spend more time testing it. Beyond that, though, Apple doesn't try to divine any greater insight into your health with this information. I haven't seen any apps try to take advantage of it yet either.

In other words, with the right support, there’s potential for this temperature system to unlock an even closer understanding of how our bodies are working. But I’m not sure anyone should rush out to buy a Series 8 right now for something it might be able to do better in the future.

The real changes are in the software

Here’s the ironic thing about living with this new Apple Watch: some of the things I’ve found most useful here aren’t actually exclusive to the Series 8. Instead, they’re baked into the company’s new WatchOS 9 update, which you can install on Apple Watches as old as 2018′s Series 4.

For people prone to atrial fibrillation like me, WatchOS now has a feature that tracks the time your heart spends beating unusually and spits outs a measure of how long it does that for in a week. (Unlike the watch’s electrocardiogram feature, which can generate a PDF version you can send to your doctor, this “AFib history” seems mostly meant for your personal enlightenment.)

WatchOS 9 also includes new sleep tracking tools, including estimates of how long you’ve spent in different sleep “stages.” Learning more about the quality of your sleep can get you get more of it, but remember: sleep experts say some of this data isn’t worth obsessing over.

You can get better sleep with wearables. Just focus on the right data.

For fitness nerds, Apple’s new software now includes a way to view your heart rate “zones” mid-workout, not to mention more in-depth insights for runners like stride length and power. And frequent wanderers may get a kick out of an improved Compass app, which can now show you a rudimentary map of where you’ve been and how to get back there.

It’s a surprisingly full bundle of software and there’s enough to dig into that people who already have usable Apple Watches should play with this update before even thinking about buying a new model.

Who it’s good for

People who have never owned a smartwatch. After seven years of fine-tuning, the Series 8 is unsurprisingly competent, and I’d recommend newcomers look at the Series 8 before the $299 Apple Watch SE.

Sure, they both do all the usual smartwatch things, like displaying notifications and letting you respond to messages. Still, the Series 8 has a better collection of sensors to monitor your well-being, and take it from me — they’re the sorts of things that don’t always feel valuable until, one day, they are.

People with some heart conditions. People who have experienced similar atrial fibrillations will find some value in having a wearable that can take reasonably accurate electrocardiograms they can share with their physicians.

Who it’s not good for

People who want a basic smartwatch. That’s not to say the new Apple Watch is terribly complicated — it isn’t. But iPhone owners who don’t find themselves fussed by health-forward features like temperature tracking or don’t need to spend Series 8 money: there’s always that cheaper SE model.

People who want multiday battery life. The Series 8 lasts for about a day and a half off a single charge — though it can go for longer if you keep the new low-power mode turned on. But Apple Watches have never been the kind of wearable you can keep on your wrist for days on end; if that’s what you’re after, one of Fitbit’s Sense watches might be a better choice.

People who bought an Apple Watch not that long ago. The first years of the Apple Watch were marked by significant annual changes. These days, the pace of progress doesn’t seem quite as fast — if you have an Apple Watch Series 6 or newer, don’t worry about upgrading just yet.

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