From the moment your head hits the pillow to your final blink at morning light, your smartwatch or ring quietly logs data — movement, pulse, breathing, even skin temperature — offering you a nightly sleep score or graph of your “sleep architecture.” But how accurate are these devices? What should you pay attention to, and what should you ignore?
As a sleep psychologist, I’m both fascinated and cautious when it comes to wearable sleep technology. For many people, it sparks motivation to prioritise rest. For others, it becomes a source of stress or misinformation. Let’s unpack what these devices can — and crucially, can’t — tell you about your sleep.
What Does Good Sleep Look Like
Before diving into what wearables measure, let’s define the goal. According to the World Sleep Society and clinical guidelines, good sleep for adults usually means:
- Total Sleep Time (TST) of 7–9 hours
- Sleep Onset Latency (SOL) (the time it takes to fall asleep) of 5–30 minutes
- Consistent sleep and wake times, even at weekends
- Brief night awakenings that you fall back to sleep from easily
- Daytime alertness, without persistent fatigue
No device can fully capture these dimensions — especially the subjective ones like how rested you feel — but some of the latest features of your wearable can come pretty close.
What Your Sleep Wearable Is Measuring (and How)
Most consumer sleep devices use a combination of sensors and algorithms to infer sleep patterns. These include:
1. Movement (Accelerometry)
Your wearable tracks movement using accelerometry — essentially measuring how fast and how often you move. Because we generally move less during sleep (especially in deep or REM sleep), it’s a useful but imperfect proxy.
Limitations:
- You might be lying still but awake (e.g. reading), leading to false positives for sleep.You might be restless while sleeping (especially children, or people with sleep disorders), which can be misread as wakefulness.
- Sharing a bed or certain physical conditions can throw off results.
2. Heart Rate (Photoplethysmography or PPG)
PPG technology uses light to detect blood volume changes, helping the device calculate your pulse. Since heart rate drops during sleep — and changes across sleep stages — it helps estimate not only sleep duration but sometimes sleep depth.
Limitations:
3. Breathing (Respiratory Rate)
Breathing is often inferred through changes in heart rate or chest movement. Some wearables use sensors to track breath rate as a marker of relaxation or potential sleep disturbances.
Limitations:
- Accuracy depends on your sleeping position, fit of the device, and whether you’re sharing a bed.
- Health conditions or altitude changes can alter respiratory rate independent of sleep.
4. Temperature
Many newer wearables use thermistors to detect tiny changes in skin temperature, which reflect internal body temperature fluctuations during sleep.
Limitations:
- Temperature is highly influenced by room conditions, illness, or bedding.
- The readings can also be significantly influenced by where the thermistor is, for example if you’re sleeping with your hand under your pillow, or against your skin.
Key Sleep Wearable Metrics — What They Mean (and Don’t Mean)
Total Sleep Time (TST)
This is the most commonly tracked and one of the most useful metrics. Your device estimates the time you were asleep, subtracting any prolonged night wakings. However, naps are often missed or miscounted, and night wakings may be missed.
Sleep Onset Latency (SOL)
Wearables attempt to measure how long it takes you to fall asleep. Good sleepers usually drift off within 5–30 minutes.
Caution: This is one of the least reliable estimates. Devices often assume you’re asleep as soon as you stop moving — even if your mind is racing. Instead of focusing on the exact number, track trends over time. 3
Night Wakings / Wake After Sleep Onset (WASO)
Brief night awakenings are normal. Good sleepers return to sleep quickly and often don’t recall them. If your wearable shows long waking periods or frequent fragmentation, it’s worth paying attention.
Watch out: Most devices need around 3 hours of continuous sleep to log it properly. If your night is broken into chunks, it may under-report your total sleep.
Sleep Stages
Many wearables claim to report time spent in light, deep, and REM sleep. While we do cycle through these stages multiple times each night, the accuracy of these estimates is limited.
Why it’s tricky: Polysomnography (PSG) — the gold standard — uses EEG to track brain activity. Wearables don’t. So they’re making educated guesses, not measurements.
Good news: If you’re sleeping long enough and feel rested, your brain will naturally regulate the right proportions of each stage. You don’t need to hack your way to “more deep sleep.”
Sleep Scores: Helpful Summary or Misleading Metric?
Most sleep wearables assign you a “sleep score” — a number representing the perceived quality of your sleep. While this can be a quick overview, this single number is a huge oversimplification of what good sleep really means. Caution: This is one of the least reliable estimates. Devices often assume you’re asleep as soon as you stop moving — even if your mind is racing. Instead of focusing on the exact number, track trends over time 3.
Limitations:
- Scores vary between brands, and there’s no universal standard.
- A “low” score might reflect a single bad night — which is not something you need to worry about.
- Obsessing over scores can backfire, causing stress and worsening sleep4 — a condition now dubbed orthosomnia.
A more promising future would see sleep scores combined with long-term health data — such as how certain patterns relate to risks for hypertension or metabolic disorders. But we’re not there yet.
Sleep Satisfaction: The Missing Metric
Some devices now let users “tag” or journal their sleep — for example, noting if they had caffeine late, travelled, or felt stressed. This is a huge step forward. Combining subjective sleep satisfaction with objective data helps identify personalised patterns and what actually helps you sleep better.
Tracking your sleep is only helpful if it leads to insight and behaviour change. You’re the expert on your sleep — not your wearable.
How to Get the Best Out of Your Sleep Wearable
If you’re using a sleep wearable (or thinking about getting one), here’s how to use it wisely:
✅ Understand what good sleep looks like for you: We all have slightly different sleep needs, so take time to learn what you need to wake up feeling fully refreshed.
✅ Look for trends, not perfect numbers: Focus on your typical pattern, not the occasional off night. Good sleep is all about patterns over time.
✅ Follow manufacturer guidelines: Getting the right placement and tightness is important for the most accurate readings.
✅ Log naps and manual corrections: If your device allows, add in missed naps or adjust mistimed nights.
✅ Know when to take a break: If your wearable is stressing you out, it’s okay to stop wearing it for a few nights.
✅ Don’t ignore red flags: Frequent alerts about breathing irregularities or fragmented sleep? Consider speaking to a sleep professional — your wearable can’t diagnose sleep apnea or insomnia, but it might point you in the right direction.
Final Thoughts: Keep It in Perspective
Wearables are an exciting and rapidly evolving tool in the sleep science toolbox. In fact, some studies suggest that consumer devices are beginning to outperform research-grade actigraphy when paired with the right algorithms.5 6 7 Their ability to track sleep across thousands of nights, across populations and settings, may transform how we understand sleep in the real world.
But at an individual level, your sleep wearable is just one piece of the puzzle. It doesn’t know if you’re worried about your job, had a fight with your partner, or are battling jet lag. It doesn’t know how you feel in the morning. And it doesn’t define your health.
If you’re concerned about your sleep — or confused by what your wearable is telling you — don’t hesitate to get professional advice. After all, understanding your sleep is about more than data. It’s about wellbeing, resilience, and reclaiming rest in a world that doesn’t always prioritise it.
If you’d like to explore your sleep data in more depth, or need support with persistent sleep difficulties, get in touch for a free 15 minute consultation to find out how we could work together to improve your sleep.
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