Wearables have solved a genuine problem. For most of medical history, the only physiological data anyone had was collected in a clinic a few times a year. Now people have continuous data on heart rate, sleep, activity, and temperature. That's a real advance.
It has also created a new problem, which is that a lot of people are now making decisions from numbers they don't understand the reliability of.
A rough reliability hierarchy
Not all wearable metrics are equally trustworthy, and the devices generally don't tell you which is which.
Steps and heart rate during steady activity are measured reasonably directly and tend to be solid. Sleep duration and timing are usually decent. Resting heart rate is reliable and genuinely useful as a trend.
Sleep stage breakdowns are estimates inferred from movement and heart rate rather than brain activity, and should be read loosely. VO2 max estimates are derived from heart rate response and carry meaningful error. Calorie burn estimates are the least reliable number on most devices, often off substantially, and I'd largely ignore them.
Composite "recovery" or "readiness" scores are proprietary algorithms combining several of the above. They can be directionally useful. They are not measurements, and treating a readiness score as an objective fact about your body overstates what it is.
Read the trend, not the day. A single morning's number is mostly noise wearing a decimal point.
The right way to use the data
Track trends over weeks, not readings over days. Almost every one of these metrics is noisy day to day, influenced by alcohol, illness, room temperature, late meals, and how the device sat on your wrist. A seven-day average carries signal. Tuesday does not.
Use the data to notice patterns you'd otherwise miss: that your resting heart rate is climbing over a month, that your sleep gets consistently worse on nights you eat late, that your HRV drops for days after a particular kind of stress. Those are actionable observations.
Where it goes wrong
There's a documented pattern of people whose sleep genuinely worsens because they're anxious about their sleep scores. There's a related pattern of people who feel fine, see a poor readiness score, and then feel worse, or who feel unwell, see a good score, and override a real signal.
Your subjective experience is data too, and in some cases it's better data than an algorithm's estimate. A device telling you you're recovered when you feel terrible is not a reason to train hard.
How I'd use one
Wearables are decision support, not decision makers. That framing does most of the work. I'd use one to catch drift in resting heart rate and sleep regularity, to notice patterns I'd otherwise rationalize away, and to make abstract things concrete enough to act on.
I wouldn't use one to override how I actually feel, to chase a daily score, or as a substitute for the small number of things that actually determine outcomes: consistent training, adequate protein, regular sleep, and managing stress. The device measures the system. It isn't the system.