Updated
Phone sleep tracking is reasonably good at the broad outline of your night, such as roughly when you fell asleep, when you woke, and how restless the night was. It is much less reliable at the detail most apps put front and center: how many minutes you spent in deep, light, and REM sleep. Studies that compare phone apps with a full sleep lab test find that they detect sleep well but sort it into stages poorly.
So use a phone tracker for patterns over weeks, not verdicts about a single night. If the app says you got 52 minutes of deep sleep last night, treat that as an estimate with a wide margin, not a measurement.
What your phone is actually measuring
A phone cannot see your brain. The gold standard for sleep, polysomnography, records brain waves, eye movements, muscle tone, breathing, and heart rhythm in a lab, and a trained scorer scores the result in 30 second segments.
A phone on the mattress or nightstand has much less to work with:
- Motion. The accelerometer picks up how much you and the bed move. Long stretches of stillness suggest sleep. Movement suggests wakefulness or lighter sleep.
- Sound, in some apps. The microphone can pick up snoring, talking, coughing, or the rustle of turning over.
- Your phone use. When you last touched the screen and when you first picked it up in the morning give useful clues about when the night started and ended.
From those signals, an algorithm makes a guess. The guess about sleep versus wake can be decent. The guess about sleep stages is an inference stacked on another inference.
What the research says
The clearest test of a phone app is still a 2015 study in the Journal of Clinical Sleep Medicine by Sushanth Bhat and colleagues. They compared the iPhone app Sleep Time with in lab polysomnography in 20 adults without sleep disorders. The American Academy of Sleep Medicine's summary of the study says the app had high sensitivity for detecting sleep, but its sleep staging correlated poorly with the lab. It underestimated light sleep by 27.9% and overestimated deep sleep by 11.1%, and measures such as sleep efficiency and time to fall asleep did not line up with the lab results in any meaningful way. It tested one app on one phone, so it does not tell you how a particular app performs today, but the pattern has held up in later work.
Dedicated sleep trackers, including wrist wearables that add heart rate and bedside devices built only for sleep, give a useful upper bound for what a phone can hope for. A 2021 study in the journal Sleep by Chinoy and colleagues tested seven of them, among them a Fitbit and two Garmin devices, against polysomnography in 34 healthy young adults over three nights. Most devices detected sleep well and several matched or beat research grade actigraphy at detecting wake. Sleep stage results were mixed, and devices did worse on nights of disturbed sleep, which are often the nights people most want to understand.
The American Academy of Sleep Medicine's 2018 position statement on consumer sleep technology sums up the state of things. It says these devices mostly lack validation against polysomnography and cannot be used to diagnose or treat sleep disorders, though the data can be a useful conversation starter with a clinician when it sits inside a proper evaluation.
What you can reasonably trust
- Rough bedtime and wake time. Within a margin, a phone can tell you when the night started and ended, especially if it also knows when you put the phone down and picked it up.
- Total time in bed and roughly how long you slept. Useful for noticing that weeknights are an hour shorter than you thought.
- Restless versus settled nights. A night with much more movement than usual often matches how you felt.
- Trends over weeks. Is your bedtime drifting later? Are weekends far longer than weekdays? Patterns across many nights are much more reliable than any single number.
What to take lightly
- Minutes of deep, light, and REM sleep. This is where phones are weakest. Day to day swings in these figures are mostly noise.
- A single night's score. A sleep score compresses several estimates into one number. It can be a handy summary, but a bad score after a night you felt fine about is not something to worry over.
- Exact time to fall asleep. Lying very still while awake looks a lot like sleep to an accelerometer.
- Anything that sounds like a diagnosis. A phone cannot tell you whether you have sleep apnea, insomnia, or any other condition.
When the tracker becomes the problem
Some people start sleeping worse because of the tracker itself. In 2017, Kelly Glazer Baron and colleagues at Rush University Medical Center described several patients in the Journal of Clinical Sleep Medicine who became so focused on hitting a perfect number that the worry kept them awake. They called it orthosomnia. The Sleep Foundation notes that it is not a formal diagnosis and that research on it is limited, but the idea rings true for many people.
If checking your score in the morning makes you anxious, look at it once a week instead, or only look at the trend chart. How you feel during the day is a better guide than any figure on the screen.
How to get better readings from a phone
- Place the phone the same way every night, on the mattress near your pillow or on the nightstand, as your app recommends. Consistency matters more than the exact spot.
- Keep it charging. A tracker that dies at 3 a.m. gives you half a night.
- Expect noise from a partner or pet. Motion sensors cannot tell whose movement they feel. A shared bed makes stage estimates even less reliable.
- Do not change your night to please the app. Sleep the way you normally do. The point is to see your real pattern.
When to talk to a doctor instead
Loud snoring with pauses or gasping, falling asleep during the day when you do not mean to, or trouble sleeping most nights for weeks are worth raising with a clinician. A phone report can be something to bring along, but the clinician will decide what testing you actually need.
How Snora handles sleep tracking
Snora tracks your night from phone motion while a sleep session runs. The report shows a sleep score, sleep stages, and a breathing figure, and every one of those is labeled as an estimate. Snora is not a medical device and does not diagnose, treat, or monitor any condition. Sleep tracking and reports covering the last 90 days are free, with no account required. Snora Plus adds an AI sleep coach and keeps records beyond 90 days.
FAQ
Are sleep tracking apps accurate?
They are fairly accurate at telling sleep from wake and much less accurate at telling sleep stages apart. A 2015 study comparing an iPhone sleep app with a lab sleep test found good sensitivity for detecting sleep but poor agreement on stages, with light sleep underestimated and deep sleep overestimated.
Can my phone really track deep sleep?
Not reliably. Deep sleep is defined by brain wave patterns that only a lab test can record. A phone infers it from how still you are, which is only loosely related. Treat deep sleep minutes from a phone as a rough estimate and look at trends rather than single nights.
Is a smartwatch more accurate than a phone for sleep?
Usually somewhat, because a watch adds heart rate and sits on your body. A 2021 study of seven consumer sleep trackers, including several wrist wearables, found most detected sleep well, but stage results were still mixed and accuracy dropped on disturbed nights. Neither a phone nor a watch replaces a sleep test when a doctor needs one.
Should I worry about a low sleep score?
Not after a single night. A score is built from several estimates, and one low number often reflects movement, a shared bed, or the phone's placement as much as your sleep. Look at the pattern over a week or two and at how you feel during the day.