After one stormy night, the rugged fitness watch I’d been wearing for months delivered bad news: I’d spent just five minutes in rapid eye movement, or REM, sleep.
The rest of the numbers didn’t seem much better. 24 minutes of “deep” sleep. Almost six hours of lighter sleep. Over an hour and a half awake and averaging about 15 breaths per minute.
That is certainly a lot of information. And that at least partially explains why I spent the next morning in a mental fog. However, as it turns out, the following days I spent agonizing over some of these numbers may have been less helpful than I thought.
According to the Centers for Disease Control and Prevention, people who routinely get less than seven hours of sleep a night are more likely to report health problems such as heart disease, diabetes and depression. Meanwhile, a recent study from the University of Galway in Ireland suggests that people with recurring sleep problems are more likely to suffer a stroke in their lifetime.
It’s no wonder, then, that wearables — the kind worn by about 20 percent of American adults — are churning out sleep-related characters to help us make sense of our time away from consciousness. Catch? Sometimes these numbers are presented without much context, which can make it difficult to understand how valuable they really are. And other numbers, like the amount 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 accurately measure what our brains are doing. Instead, it does its best to guess where we are in our nightly sleep journeys by tracking and interpreting the kind of data a watch or ring can collect, like your heart rate and movement in bed.
“They are a proxy for sleep, not sleep as traditionally defined,” said Cathy Goldstein, a sleep researcher and associate professor of neurology at the University of Michigan.
None of this means you shouldn’t try to use your wearable technology to better understand how you sleep. Goldstein says these kinds of gadgets can be really useful because “otherwise we don’t have a way to track sleep over days and days.” Trick? Pay attention to the right kinds of data. To help you out, here’s our guide to the sleep-related numbers your wearable tech might be throwing up at you, and how seriously you should take them.
– Total sleep time
Tracking it without a smartwatch or a fancy ring is simple enough in theory: note when you went to bed and when you woke up, then do a little math. But where wearables really come in handy is getting the full picture of your time in bed.
“Almost all commercial trackers now are really good at telling you when you went to bed, when you woke up, how long you slept and how awake you were,” said Joshua Hagen, director of the Human Performance Collaborative at Ohio State University.
Ideally, he says, you should aim for seven to nine hours of real sleep a night—which is quite different from spending seven to nine hours in bed trying to nod off. If, like many of us, you don’t hit that watermark, seeing these numbers on your smartphone can help you recognize that your sleep habits need fixing.
“It’s kind of like tracking your calories,” says Goldstein. “It doesn’t change anything, but it makes you recognize the problem.”
Verdict: This is the most useful number to pay attention to.
– Time in different stages of sleep
“The things I warn my patients not to get upset about are certain times spent in REM sleep or deep sleep,” Goldstein said.
When professionals conduct studies to properly delve into sleep quality, she says, they rely on sensors that directly monitor brain activity, eye movement, chin and leg muscle movement, and much more. Only after the researchers have collected all of this measured data for the entire night do they go back and determine, say, how long someone spent in each stage of sleep.
Meanwhile, the most popular, commercially available wearable gadgets track only a few of these signals. And none of them can predict 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 sleep stages appear on electroencephalograms. “We simply cannot expect [wearables] measure the same.”
What’s more, it’s possible to read too much into some of these sleep stage numbers. Goldstein says researchers often 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 significance of these changes.”
And besides, Ohio State’s Hagen says there isn’t a lot of definitive information on how to extend deep sleep, so it’s not really worth stressing over that number.
“There’s not much you can do about it,” he said. “Your body will get what it needs.
Verdict: Take these numbers with a grain of salt.
– Heart rate variability
If your heart rate is 60 beats per minute, it doesn’t beat exactly once per second – there are micro differences between these lubes and dubs. Together, these small variations make up the variability in your heart rate, which Hagen views as a “global stress indicator,” measured in milliseconds. And perhaps paradoxically, the higher your HRV, the better.
“If you’re very emotionally stressed, it’s very likely that you could have low heart rate variability,” he said. “If you are sick, you can also have a low HRV. If you are rested and relaxed and everything is good in life, you will most likely have a higher HRV compared to your norm.”
The data about the weakest fluctuations of your heart sounds quite esoteric, and it is true that you could do well without thinking about it. But the University of Michigan’s Goldstein says the number can be useful for getting an idea of the impact of some of the things you do in your daily life on the quality of your rest.
“If you’ve been drinking, if you’ve eaten certain foods, you can have changes in heart rate variability,” she said.
These drops in HRV at night can help you get rid of habits and practices that you should skip during the day. And checking your HRV during the day can also give you a clearer picture of how restful – or not – last night was.
Verdict: You probably don’t need to watch this all the time, but it can be informative.
– Breaths per minute
Wearable devices like smartwatches and rings are surprisingly good at measuring our breathing. But do most people really need to know how many times they breathe per minute while they sleep?
That depends on how much context you have.
“For the average consumer, looking at your respiratory rate every day probably doesn’t give you a lot of information,” says Hagen. But tracking this number and how it changes over time can offer key insights into the quality of your sleep.
During sleep, most people tend to move between 12 and 20 breaths per minute, and changes in this breathing rate can signal serious problems. (A persistently low breath-per-minute count while sleeping can be a sign of sleep apnea, for example.) But the name of the game, according to Hagen, is to keep your eyes peeled for consistent deviations from your norm—whatever that may be.
“Everybody’s numbers will be specific,” he said. “The more you understand, the better the data can be used.”
Verdict: Worth keeping an eye on over time, but comparing to others may not be helpful.
GETTING THE RIGHT TRANSFER
Tracking 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.
– Universal wearable device: Apple Watch. If you’re one of the many people who use an iPhone, the Apple Watch can quickly start providing sleep data that could help you.
Of all the models the company sells, the Apple Watch Ultra offers by far the best battery life — which is why I’m using one now — but otherwise it’s frankly overkill unless you’re a serious athlete. Fortunately, you don’t need the latest or most expensive model; even older models dutifully monitor sleep time and heart rate variability.
– A more subtle option: Oura Ring Gen 3. I got some of the most immediately useful sleep information from the latest Oura Ring ($299 plus a $5.99 monthly subscription), which packs a lot of sensors into a sleek ring design. That said, it’s a bit sturdier than many other rings I’ve worn over the years.
Its companion app is some of the best I’ve seen at presenting the data it captures, and I really enjoyed how unobtrusive the ring itself is, but not everyone will like the idea of paying monthly to use a gadget they’ve already spent on. hundreds of dollars, even though the first month of service is free.
– Another discrete option: WHOOP 4.0. A number of readers wrote to the Help Desk asking me to try the WHOOP, a nondescript fitness band that you’re supposed to wear on your wrist or arm. I’m glad they did – it takes a fascinating approach to tracking your movement, aimed at measuring the amount of stress you put your body through and how well you recover from it.
Its sleep trackers offer many of the same metrics as other devices, though thankfully the WHOOP app highlights certain kinds of data — like how many times you wake up — in more detail. It also offers suggestions for how much sleep you want per night based on how hard you pushed your body that day.
That said, the WHOOP experience takes a lot of getting used to. First, you don’t buy the band outright—instead, you pay a monthly fee for 24/7 access to the service and get the wearable for “free.” And unlike other devices that you plug in and charge normally, you have to charge a separate “backpack” battery that you attach to the WHOOP band when it needs to refuel.
