How Do Sleep Trackers Work + Are They Accurate?

From sleep cycles to blood oxygen levels, REM sleep to non-REM sleep, there’s a lot to consider when it comes to getting high-quality shut-eye. That’s why many have recently turned to sleep tracking for insights into what’s going well, what’s not, and how to improve things. But with so many trackers to choose from, it’s difficult to know where to start. Not every device tracks the same metrics—and even if they do, it’s likely that the way the metrics are tracked differs device-to-device. To help you make the best decision, we dug into the five most popular trackers: Fitbit, Garmin, Apple Watch, Oura Ring, and Whoop.

A few notes on the way we went about our sleep tracker analysis:

• We focused on wearables that track sleep habits, but know that there are also non-wearable trackers such as Withings Sleep.
  
  
• Proper is not in any way paid to feature these brands—or to prioritize one over another. We’re just here to dive into the science behind how each works and how accurate it is so you can select the best sleep tracker for you.
  
  
• Electroencephalogram (EEG) sensors used in overnight sleep studies are the gold standard for assessing sleep stages. Without this data, what you’re getting with sleep trackers is an estimate based on movement, heart rate, and other parameters.

Best for: Advanced sleep and fitness tracking

What it is: Wristband or watch (Fitbit Sense)

What it tracks

• Total sleep duration
• Time you spend in light, deep, and REM sleep (sleep stages/sleep phases)
• Time you spend awake or restless
• Heart rate

On the Fitbit Sense (their most advanced health smartwatch), you’ll also be able to track oxygen saturation in addition to downloading a compatible ECG app to assess heart rate irregularity. Plus, the Sense comes with a six-month Calm subscription, the #1 app for meditation and relaxation (which both facilitate better sleep).

The different sleep stages are estimated using a combination of your body movement and heart rate patterns. For example, when you haven’t moved for about one hour, your Fitbit logs that you’re asleep. From there, your device tracks beat-to-beat changes in your heart rate—also known as heart rate variability, or HRV for short—which fluctuates as you progress between light sleep, deep sleep, and REM sleep stages.

Combined, these metrics generate a daily sleep score to help you understand trends in the quantity and quality of sleep each night.

Note: The above metrics are tracked on the basic subscriptions, but you’ll need to upgrade to a premium subscription to get:

• Analysis of your sleeping heart rate and restlessness
• Snore and noise detection
• Sleep duration analysis

Additional features

• Sleep mode
  So you can mute notifications and prevent your screen from turning on at night
  
  
• Smart alarm
  So an alarm can go off during your optimal stage of sleep (within 30 minutes prior to its set time)
  
  
• Stress management score
  So you better understand your body’s physical response to stress and how certain factors (sleep, mindfulness, workouts) impact it
  
  
• Daily readiness score
  Reveals whether you’re ready to work out or should prioritize recovery (only on Premium)
  
  

In terms of battery life, you’ll get up to seven days on the Fitbit Charge 5 and six days on the Fitbit Sense.

How to read and interpret the data

Your sleep score ranges from one to 100. According to Fitbit, most receive a score between 72 (fair) and 83 (good).

Here’s how to interpret your score:

• 90 - 100: Excellent
• 80 - 89: Good
• 60 - 79: Fair
• < 60: Poor

Some may also have access to a “restoration” score, which indicates how restful your sleep was. This is based on both your resting heart rate and how much time you spent tossing and turning. Higher scores mean your sleeping heart rate is lower than your daytime resting heart rate.

How accurate is Fitbit sleep tracking?

According to a 2019 systematic review published in the Journal of Medical Internet Research, (1) Fitbit models showed promising performance in differentiating wake time from total sleep time, as well as time spent in sleep stages; however, they are of “limited specificity and are not a substitute for [polysomnography] PSG,” which is conducted as part of a sleep study in a dedicated sleep lab (where you can also track brain activity with an EEG (electroencephalogram).

READ MORE: A Guide To The 4 Stages Of Sleep (REM + NREM)

Best for: Advanced sleep tracking

What it is: Watch

What it tracks

• Total sleep duration
• Average stress score during sleep
• Time you spend in light, deep, and REM sleep (sleep stages)
• Time you spend awake or restless
• Heart rate
• Oxygen saturation (pulse oximetry)
• Respiration (your breathing patterns as measured by how often you inhale and exhale in a minute...aka your breathing rate)

Combined, these metrics generate your nightly sleep score, which is calculated based on how long you slept, how well you slept, and evidence of recovery activity in your autonomic nervous system (which is derived from heart rate variability).

How to read and interpret the data

Like Fitbit, Garmin’s sleep score is calculated on a scale of 1-100 and should be interpreted as follows:

• 90 - 100: Excellent
• 80 - 89: Good
• 60 - 79: Fair
• < 60: Poor

How accurate is Garmin sleep tracking?

In June 2019, Garmin released results from a study (2) conducted under the supervision of Dr. Suzanne Stevens, Director of the University of Kansas Medical Center Sleep Medicine Clinic. There were 55 participants, 14 of whom indicated that they have a sleep disorder or take medications that may impact their sleep architecture. 31% were female and 69% were male, with an average age of 33.5 +/- 8.7 years. Results showed that the overall accuracy against real-world data was 69.7%, with a 95.8% sensitivity in detecting sleep and 73.4% specificity in detecting awake.

Best for: Basic sleep tracking

What it is: Watch

What it tracks

With the free Sleep app on your Apple Watch (available for WatchOS 7 or later on Apple Watch Series 3 or later), you can easily set what’s called “bedtime schedules” to help you meet your goals. Included in the sleep schedules are:

• Sleep goal (i.e. how many hours you want to get per night)
• What time you want to go to sleep
• What time you want to wake up (along with your preferred alarm sound)
• Average time you spend in bed
• Average time you spend asleep

Additional features

When it’s almost time for bed, the Sleep app will activate “Wind Down” mode, which will automatically turn off the display screen on your smartwatch and trigger “Do Not Disturb” mode.

How to read and interpret the data

The data on an Apple Watch via the Sleep app is fairly straight-forward and more simplistic than what you’ll find on other types of sleep trackers, so there’s not much that goes into interpreting it. If you're looking for more detailed sleep metrics, it’s best to download other third-party smartphone apps (although keep in mind that you may need to pay for some of them).

How accurate is Apple Watch sleep tracking?

The accuracy of the Apple Watch is difficult to gauge (3) since Apple itself does not promote any of its watches as having sleep monitoring capabilities. Instead, it’s up to the third-party apps to address this need. The problem? The scientific evaluations around these third-party apps are either incomplete (i.e. lacking peer review) or nonexistent.

Best for: Advanced sleep tracking

What it is: Ring

What it tracks

There are three daily scores you’ll get with an Oura Ring:

1. Sleep score
   Calculated based on your quantity of deep sleep, REM sleep, and light sleep, as well as your nightly heart rate and bedtime schedule.
   
   
2. Activity score
   Calculated based on your daily movement/inactivity and how much rest you’re getting at night.
   
   
3. Readiness score
   Calculated based on your sleep, activity, and body stress signals (e.g., body temperature and HRV) to help you understand how “ready” your body is to take on more activity.

Additional features

There are a few additional features worth noting with an Oura Ring, including:

• Restorative time
  Based on your heart rate and skin temperature, the Oura Ring can pick up on when you’re in a relaxed state so you can better understand whether or not you’re taking enough recovery breaks throughout the day.
  
  
• Nap detection
  If you’re pro-napping and want to get credit for that shut-eye time, the Oura Ring will capture it.
  
  
• Rest mode
  Based on your key vitals, the Oura Ring will detect whether you’re tired, unwell, or under an outsized amount of stress. From there, it automatically adjusts your daily goals to prioritize more rest and recovery.
  

Unlike other wearable devices like the Apple Watch that need to be charged daily, the Oura Ring has a battery life of about seven days.

How to read and interpret the data

Over time, Oura will learn about your sleep and, based on your nights of high quality or low quality sleep, will share personalized guidance and insights on things like when to start winding down.

How accurate is Oura Ring sleep tracking?

In January 2020, Oura published results of an internal study of 49 healthy subjects (4) which showed that, compared to a medical-grade electrocardiogram device (ECG), the Oura ring performed near-perfect for resting heart rate and (RHR) and heart rate variability (HRV). Additionally, their new sleep staging algorithm—which was released in June 2021—was found (5) to achieve 79% agreement with gold-standard PSG for identifying sleep stages (wake, light, deep, and REM).

Best for: Advanced sleep tracking + exercise recovery

What it is: Wristband

What it tracks

• Time spent in bed + time spent asleep
• Sleep stages + cycles (awake, light sleep, REM sleep, deep sleep)
• Sleep disturbances
• Respiratory rate based on number of breaths per minute
• Sleep efficiency
• Sleep latency (how long it takes you to fall asleep)

Additional features

• Baseline sleep needs
  Because not everyone needs the same amount of sleep, Whoop also provides insight into your ideal sleep quantity based on your unique physiology and recent trends. Whoop can also detect when you have strenuous days, which require more sleep to recover. 
  
  
• Sleep debt
  Whoop monitors your sleep trends and will let you know how much sleep you’ve missed over the course of a night or several nights so you can aim to correct this. 
  
  
• Nap detection
  Whoop will track your naps, then subtract that time from your total sleep need at night. 

How to read and interpret the data

A big advantage of Whoop is the “sleep coach” section. Here, you’ll get personalized recommendations based on your data, which can help you better understand what the numbers mean for you and what steps to take to improve.

For example, based on how strenuous your day is and when you need to wake up the following day, Whoop will adjust its sleep need suggestions. This way, you can see the number of hours you actually slept side-by-side with the number of hours Whoop suggested that you sleep.

How accurate is Whoop sleep tracking?

In a third-party study conducted at the University of Arizona Health Sciences Center for Sleep and Circadian Sciences, (6) researchers evaluated the WHOOP Strap 3.0 to determine its efficacy against polysomnography (PSJ), which records brain waves, blood oxygen levels, heart rate, breathing, and eye/leg movements during sleep.

Results were promising, with a study (7) showing that there was high correlation between WHOOP differentiation of wakefulness and sleep. However, when it came to distinguishing the different stages of sleep, WHOOP was not as sensitive “to light sleep, SWS, REM, and wake, and Cohen's kappa [used to measure reliability] were 62%, 68%, 70%, 51%, and 0.47, respectively.”

• If you’re looking for basic sleep tracking: Apple Watch
• If you’re looking to dive deep into the science of sleep: Oura Ring, Garmin
• If your goal with sleep tracking is to optimize exercise performance and recovery: Fitbit, Whoop

With so much information floating around about sleep—not to mention all the data thrown at us from wearable sleep trackers—it’s hard to know what good quality sleep looks like. The problem then is that we may be either too quick to self-diagnose or to dismiss things like sleep deprivation or other sleep problems, which can inhibit our ability to get better sleep.

In our full guide to “normal” sleep, two of Proper’s expert sleep coaches, Kelly O’Brien and Lauren Hoogs, weigh in on what normal sleep should look like, including:

• Normal sleep cycle
• Normal oxygen levels during sleep
• Normal sleeping heart rate
• Normal amount of deep sleep
• Normal amount of REM sleep
• Normal amount of light sleep
• Normal overall quantity of sleep

​​“It’s been said that the truth is in the tracking,” explains O’Brien. “Important details around certain behaviors can go unnoticed or seem insignificant. When we pay closer attention, we can gain powerful insights.”

Hoogs is in agreement.

“Sleep data gives us the opportunity to find patterns we may not be conscious of yet. There are so many factors that impact our sleep patterns and sleep quality, and while some may be obvious, some are more insidious. The more detailed we are with our sleep monitoring, the more we can understand and peel back the layers that are impacting our sleep so we can get the sleep we need.”

However, the benefits are only gained with intentional, healthy tracking practices. Here’s what that entails and why it’s so important:

• Remaining neutral and non-judgmental about the data
  “If we become self-critical or assign too much meaning to the data, it can cause activation or agitation that has a negative impact on our sleep quality,” says O’Brien.
  
  
• Not letting data overpower what your body is telling you
  “I meet with plenty of people who look at their data and come up with conclusions that are totally out of line with what their body is actually telling them,” says Hoogs. “Your body will tell you what it needs if you listen, and data can make us deaf to that.” Plus, it’s important to remember that there’s always the possibility of a standard error of measurement.
  
  
• Creating personal “rules of engagement” with tracking
  “Some find tracking in cycles helpful (X days on and X days off),” explains O’Brien. “Others might only check the data if they feel the night of sleep was less than ideal. Still others find that any tracking can be too activating, and they start overthinking all things related to sleep. It’s important to get a sense for what strategy might be the best fit for you.” 

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