LodestarYour Fitbit is pretty good at knowing when you're asleep and how long for. It is less precise about which stage you're in at any moment. Knowing the difference makes the data much more useful.
How Fitbit tracks sleep
The gold standard for measuring sleep is polysomnography, an overnight lab study that records brain waves, eye movements, muscle tone, breathing and heart rhythm. Sleep stages are defined by those brain wave patterns.
A Fitbit can't see brain waves. It uses movement from its accelerometer and your heart rate, including how much your heart rate varies, and estimates the stage from those signals. Stages do have physical signatures. Deep sleep comes with very low, steady heart rate and almost no movement. REM comes with a more irregular heart rate and a still body. Those patterns are what the algorithm uses.
What it gets right
Total sleep time is the strongest number. Studies comparing consumer trackers with lab sleep studies have generally found that wrist devices are good at telling sleep from wake across the night. One study tested seven consumer sleep trackers against lab polysomnography and found most performed well for sleep versus wake, with more variation on stages (Chinoy and colleagues, Sleep, 2021).
Night-to-night changes in your own sleep are also useful. Even if the stage totals are a bit off, they tend to be off in a consistent way for you, so trends over weeks are meaningful.
Where it's weaker
- Lying awake quietly. If you are still and relaxed but awake, a tracker can count some of that as light sleep. People with insomnia often find their tracker is more generous than they feel.
- Individual stage minutes. On a single night, the split between light, deep and REM can be noticeably different from a lab result. Light sleep tends to absorb the uncertainty.
- Short awakenings. Brief wake-ups happen to everyone many times a night. Trackers catch some and miss others.
- A loose band. Heart rate readings get worse if the device slides around, and stage estimates depend on them.
How to read your data with this in mind
- Trust total sleep time and the timing of your sleep the most.
- Use stages as a trend. A week of low deep sleep means more than one night.
- Compare with yourself, not with charts of "ideal" stage percentages or with other people.
- Pair the data with how you feel. If the tracker says you slept well and you feel awful for days, believe your body and look into it.
Don't let tracker numbers keep you up at night either. Some people get anxious chasing a perfect sleep score, which doesn't help sleep. If you've had trouble sleeping most nights for weeks, a doctor is a better next step than a better device. Insomnia, sleep apnea and other sleep problems need proper assessment.
How Lodestar helps
Lodestar is an iPhone app that reads your Fitbit sleep sessions, stages and naps through Google Health. It leans on the parts of the data that hold up best: your Sleep score starts from how long you slept against your own sleep need, with deep sleep, REM and efficiency adding detail. It also tracks sleep debt and sleep regularity, which are built on sleep timing and duration, the strongest parts of the signal. Everything is worked out on your phone.