After one fitful night last week, the chunky fitness watch I’ve been wearing for a few months delivered some bad news: I had only spent five minutes in rapid eye movement, or REM, sleep.
The rest of the numbers didn’t seem much better. 24 minutes of “deep” sleep. Close to six hours of lighter sleep. More than an hour and a half awake and an average of about 15 breaths per minute.
That’s a lot of information, certainly. And it at least partly explains why I spent the following morning in a mental fog. As it turns out, though, the subsequent days I spent agonizing over some of those numbers might have been less helpful than I thought it was.
According to the Centers for Disease Control and Prevention, people who routinely sleep less than seven hours a night are more likely to report medical conditions such as heart disease, diabetes and depression. It’s no wonder, then, that wearable gadgets — such as those worn by about 20 percent of American adults — spit out sleep-related figures meant to help us understand our time away from consciousness. The catch? Sometimes those numbers are presented without much context, which can make it tricky to understand how valuable they really are. And other numbers, like the amounts of time you spend in restorative “deep” sleep, are in some ways just educated guesses.
That’s because, despite how sophisticated some of these wearables have become, they can’t precisely measure what our brains are doing. Instead, they do their best to estimate where we are in our nightly sleep journeys by tracking and interpreting the sort of data a watch or a ring can collect, like your heart rate and movement in bed.
“These are a proxy for sleep, not sleep as traditionally defined,” said Cathy Goldstein, a sleep researcher and associate professor or neurology at the University of Michigan.
None of this means you shouldn’t try to use your wearable tech to better understand the way you sleep. Goldstein said these kinds of gadgets can be really helpful because we “don’t otherwise have a way to track sleep over time for days and days.” The trick? Pay attention to the right kinds of data. To help, here’s our guide to the sleep-related numbers your wearable tech could spit out at you, and how seriously you should take them.
Total time asleepKeeping track of this without a smartwatch or a fancy ring is simple enough in theory: make note of what time you went to bed and when you woke up, then do a little arithmetic. But where wearables come in really handy is getting a full picture of your time in bed.
“Almost all of the commercial trackers are now really good at telling you what time you went to bed, what time you woke up, how much sleep you got and how awake you were,” said Joshua Hagen, director of the Human Performance Collaborative at Ohio State University.
Ideally, he said, you should aim to get between seven and nine hours of actual sleep a night — a far different thing from spending seven to nine hours in bed trying to nod off. If you fall short of that watermark, like many of us, seeing those numbers spelled out on your smartphone can help you recognize that your sleep habits need fixing.
“It’s kind of like when you track your calories,” Goldstein said. “It doesn’t change anything, but it gives you recognition of the problem.”
The verdict: This is the most immediately helpful number to pay attention to.
Time in different sleep stages“The things I caution my patients not to get upset about are particular amounts of times spent in REM sleep or deep sleep,” said Goldstein.
When professionals perform studies to properly dig into the quality of a person’s sleep, she said, they rely on sensors that directly monitor brain activity, eye movement, chin and leg muscle motion, plus much more. It’s only after researchers have collected all those readings over a full night that they go back and make determinations about, say, how long someone spent in each sleep stage.
Meanwhile, most popular, commercially available wearable gadgets track just a few of those signals. And none of them can surmise what’s going on in your brain as accurately as the electrodes that would be stuck to your scalp during a sleep study
“These are states that are defined by their EEG constructs,” she said, referring to the way phases of sleep appear in electroencephalogram readings. “We just can’t expect [wearables] to be measuring the same thing.”
What’s more, it’s possible to read a little too much into some of these sleep stage numbers. Goldstein said researchers frequently don’t measure time spent in REM or deep sleep for more than a few days at a time, so they “don’t really know the relevance in the changes.”
And beyond that, Hagen from Ohio State said there isn’t a whole lot of definitive information about how to increase your deep sleep time, so stressing over that number isn’t really worth it.
“There’s not much you can do about that,” he said. “Your body is going to get what it needs.”
The verdict: Take these figures with a grain of salt.
Heart rate variabilityIf your heart rate is 60 beats per minute, it doesn’t beat precisely once per second — there are micro-scale variations in between those lubs and dubs. Collectively, those little deviations make up your heart rate variability, which Hagen looks at as a “global stress indicator” that’s measured in milliseconds. And perhaps paradoxically, the higher your HRV, the better.
“If you’re emotionally super stressed out, it’s very likely you could have a low heart rate variability,” he said. “If you’re sick, you could also have a low HRV. If you’re rested and relaxed and everything’s good in life, you’ll most likely have a more elevated HRV compared to your norm.”
Data about the faintest fluctuations of your heart sounds pretty esoteric, and it’s true that you could get by just fine without every thinking about it. But Goldstein from the University of Michigan said this number can be handy for getting a sense of the toll some of the things you do in your daily life have on the quality of your rest.
“If you drank, if you ate certain foods, you may have changes in your heart rate variability,” she said.
Those dips in HRV at night may help you suss out habits and practices you should cut out during the day. And checking out your HRV during the day could also give you a clearer understanding of how restful — or not — last night was.
The verdict: You probably don’t need to watch it constantly, but it can be enlightening.
Breaths per minuteWearables like smartwatches and rings have gotten surprisingly good at measuring our breathing. But do most people really need to know how many times per minute they breathe while they sleep?
That depends on how much context you have.
“To the general consumer, looking at your respiration rate every day probably isn’t going to give you a lot of information,” said Hagen. But keeping an eye on this number, and the way it changes over time, could offer key insights into the quality of your sleep.
While sleeping, most people tend to hover between 12 and 20 breaths per minute and changes in that rate of respiration could signal serious issues. (A consistently low string of breath-per-minute readings while asleep could, for example, be a sign of sleep apnea.) But the name of the game, according to Hagen, is keeping your eyes peeled for consistent deviations from your norm — whatever that might be.
“Everybody’s numbers are going to be specific to them,” he said. “The more understanding you have, the more actionable that data could be.”
The verdict: It’s worth keeping an eye on this over time, but comparing with others might not be helpful.
Getting the right gearKeeping an eye on the right sleep stats can help you understand why you feel the way you do in the morning, but not all wearables are created equal. Here are the gadgets I’ve personally been using lately to help me track my time away from the waking world.
The battery champ: Garmin Fenix 6x Pro
It’s enormous, it’s expensive and its strap drives me crazy. Even so, I keep coming back to this $699 Garmin watch even though I’m not all that into exercise or the outdoors. That’s for a few reasons: It offers a solid selection of the important sleep stats mentioned, plus the ability to add notes to its nightly sleep records. Most importantly, it has battery life that’s measured in weeks, not days, so you won’t need to worry about losing sleep data if you forget to charge it.
The general-purpose wearable: Apple Watch
If you’re one of the many people who use an iPhone, an Apple Watch can quickly start serving up some sleep data you might find helpful. You don’t need the newest one, either — I haven’t upgraded my Apple Watch since 2019, and it still dutifully tracks my time asleep and heart rate variability.
A few things to keep in mind: Apple’s Health app is pretty good about putting some of that sleep data in context, but seeing all of the stats we’ve discussed involves bouncing between different sections of the app, which can get a little annoying. And the least expensive Apple Watch worth using isn’t the $199 Apple Watch Series 3, but the $279 Apple Watch SE. Apple still sells the former brand new, but the hardware is getting pretty old and reports of issues aren’t uncommon.
The more subtle option: Oura Ring Gen 3
I’ve gotten some of the most immediately useful sleep insight from the latest Oura ring ($299 plus $5.99/month subscription), which crams a bunch of sensors into a classy ring design. That said, it’s a little chunkier than many other rings I’ve worn over the years.
Its companion app is among the best I’ve seen at presenting the data it captures, and I’ve come to really enjoy how unobtrusive the ring itself is, but not everyone will love the idea of paying monthly to use a gadget they already spent hundreds of dollars on, even if the first six months of service are free.