Millions of people now begin the day by looking at their wrist and checking their sleep quality instead of anything else. Various smart devices, including rings and sleep trackers, claim to tell you how long your restful hours were, how deep your rest was, and if your body has replenished your energetic reserves.
It’s easy to interpret these digits as the truth without question. But, are they really reliable? And could they offer any help if you are tired despite having good sleep? The following is a summary of the studies published in non-technical language.
What Do Sleep Trackers Actually Measure?
Most consumer sleep trackers don’t measure sleep directly. Instead, they estimate it, mainly by combining:
- Movement, using a motion sensor (accelerometer)
- Heart rate and heart rate variability, using a light sensor on your wrist or finger
- Sometimes skin temperature, breathing rate or blood oxygen
From those signals, an algorithm guesses whether you’re awake or asleep and which stage of sleep you’re in. The algorithm is different in every brand, and most companies don’t share how it works.
The gold standard for measuring sleep is a sleep study (polysomnography), which records brain waves, eye movements, breathing and more in a clinical setting. Trackers don’t measure brain waves, which is the main reason they can’t match it.
What Trackers Do Reasonably Well
Studies that compare trackers to sleep study results find some real strengths:
Telling sleep from wake. Most modern devices are good at detecting when you’re asleep. In many studies, they correctly identify sleep around 85% to 95% of the time.
Estimating total sleep time in healthy adults. In people with normal sleep, several popular devices gave sleep duration estimates that were similar to a sleep study.
Showing patterns over time. Even if a single night isn’t perfect, trackers can show your general habits: when you go to bed, when you wake, how consistent your schedule is, and how your sleep changes across weeks.
Tracking heart rate and some vital signs. Resting heart rate and similar measures are generally decent in good conditions.
Where Trackers Struggle
This is where it’s worth being cautious.
Sleep stages. Most trackers claim to tell you how long you spent in light, deep and REM sleep. But studies suggest they get stages right only about two-thirds to three-quarters of the time, and results are very variable. Without brain wave data, it’s mostly educated guessing. One review noted that sleep stage data is the least reliable thing trackers report, and isn’t clinically important for treating sleep disorders.
Detecting wake during the night. Trackers have high sensitivity for sleep but lower accuracy for wakefulness. If you lie still and awake, they can mistake it for sleep. That means they tend to overestimate how long you slept and underestimate how often you were awake.
Poor or broken sleep. This effect is stronger in people with insomnia or fragmented sleep. If you’re awake a lot at night, the tracker may still show a decent night.
Different devices disagree. Wear two trackers on the same night and you might get different numbers, because each one uses different sensors and formulas.
Irregular schedules. If you work shifts or nap in odd ways, the algorithms (which are usually built around normal night sleep) may misjudge your sleep.
What About Sleep Scores?
Many apps combine your data into a single “sleep score.” These scores can be motivating, but they’re not standardised. One brand’s 85 doesn’t mean the same as another’s, and there’s no medical definition of a “good” score. Treat them as a rough guide, not a verdict.
The Downside: When Tracking Makes Sleep Worse
Some people become so focused on getting a perfect score that it causes stress and affects their sleep. This has a name: orthosomnia, the anxiety that comes from trying to achieve an optimal sleep score.
Signs you might be overdoing it:
- You feel stressed when your score is low
- You trust the score more than how you actually feel
- You go to bed worrying about your data
- You check your numbers repeatedly
If that sounds familiar, it’s okay to take a break from tracking.
What Trackers Can’t Tell You
This part is the most important. Sleep trackers are not medical devices. A tracker won’t be able to identify a sleep problem; it cannot tell you if you have the following:
- sleep apnoea
- narcolepsy
- insomnia disorder
- restless legs syndrome
- shift work sleep disorder
In addition, it will not determine why you get tired. If you’re a bit tired during the day, your tracker claiming “excellent sleep” won’t make you realize it is the right thing. Besides, if your tracker shows you had a terrible night but you still are happy and energized from inside, worry only slightly. Your subjective feelings will be the ultimate factor more than the count.
Breathing disturbance or blood oxygen level features might be incorporated with some devices These might be helpful ways of prompting you to see a doctor although they are not to be understood as a diagnosis, and a standard reading does not definitely rule out sleep apnoea. The best method of testing would be a sleep study that can be organized via your GP.
How to Use a Sleep Tracker Wisely
If you enjoy using one, here’s how to get the most out of it:
- Look at trends, not single nights. Averages over a week or two are far more meaningful than one bad night.
- Focus on consistency. Bedtime, wake time and total sleep time are the most reliable and useful numbers.
- Treat sleep stages with caution. Don’t worry if your “deep sleep” is lower than someone else’s.
- Trust how you feel. Energy, mood and concentration during the day are better measures of good sleep than a score.
- Use it to learn. Notice what affects your sleep, such as late caffeine, alcohol, late meals or stress.
- Wear it properly. A snug fit and charged battery give better data.
- Take a break if it’s stressing you out. There’s nothing wrong with putting it away for a while.
Trackers for Shift Workers
If you are a night or rotating shift worker, a tracker will give you insights into your total sleeping hours, sleep pattern consistency with reference to different shifts, and even reveal patterns like short sleeps before swing shifts or too much time spent awake after a night shift. Remember, sleep trackers are primarily made to analyze typical night sleep and may not reflect your actual sleep perfectly. So, take their results as a rough estimate. You may find our articles on how to deal with sleepiness after night work or changing shift arrangements very informative.
A Practical Tip for Your GP Visit
If you are concerned about whether you’re getting enough sleep or not, a tracker’s data can be a good initial step. It is particularly so if you’ve been wearing the tracker for a few weeks. Along with it, give your GP a simple sleep diary and a record of your symptoms (e.g., snoring, morning headaches, or daytime sleepiness). This way, your GP can recommend a sleep test if necessary.
When to See Your GP
Don’t wait for a tracker to tell you something’s wrong. See your GP if you:
- Feel very sleepy during the day, even when your tracker says you slept well
- Snore loudly, gasp or choke at night
- Wake with headaches or a dry mouth
- Have nodded off while driving
- Feel persistently exhausted for weeks
Our guide “Always Tired? How to Tell If It’s Excessive Daytime Sleepiness” explains how to prepare.
The Bottom Line
Sleep trackers can reveal habits and patterns of sleep through their monitoring capabilities and they’re pretty good at identifying the difference between the two states, that is, being asleep or awake.
However, when it comes to measuring specific stages of sleep as well as episodes of sleep interruption during the night, these devices are much less reliable. Moreover, they don’t have the capability for doing any kind of diagnoses. Therefore, treat them like a map instead of a rule-book.
If one notices that one is feeling fatigued more than usual, snoring excessively or has fears of not being able to breathe freely even when asleep, then having a device that is more advanced will not help that situation.
You need to talk it out with your GP to have it resolved once and for all.
This article is general information only and isn’t medical advice. If you’re concerned about your sleep or ongoing tiredness, speak with your GP.