Your watch or ring gives you a sleep score each morning, out of 100, and it feels authoritative because it is a number. It is worth knowing what produced it.
No consumer device measures sleep directly. The clinical method, polysomnography, records brain activity, eye movement and muscle tone, and it needs electrodes. Your wearable has an accelerometer, a heart rate sensor and sometimes skin temperature. From movement and heart rate variability it infers whether you are awake, and then infers which stage of sleep you are probably in.
Those two inferences are not equally good. Detecting sleep versus wake from movement is a mature technique and reasonably accurate, though it tends to score lying still while awake as sleep, which is why a bad night sometimes gets a decent total. Assigning stages is much harder, and independent studies have repeatedly found consumer devices land in the right stage substantially less often than the confident bar chart suggests. Deep sleep in particular moves around between devices, and between firmware versions of the same device.
So what is the number good for?
It is good as a trend about itself. Your score this week against your score last month, on the same device, with the same firmware, is a fair comparison and often a genuinely useful one.
It is good at catching the obvious. Late caffeine, alcohol, a hot room and a late heavy meal show up clearly in most people's data, and seeing it is more persuasive than being told it.
It is good at total time, roughly, and at bedtime consistency, which is the single metric with the strongest link to how you feel and the one most people ignore in favor of the stage breakdown.
It is not good for comparing against another person, against another brand of device, or against a clinical threshold. And a score that drops for no reason is far more likely to be a sensor having a bad night than your body having one.
One genuine warning sign is worth separating from all this: loud snoring with pauses in breathing, daytime sleepiness that does not respond to more time in bed, or a device flagging repeated oxygen dips. Those are worth a doctor, and the score is not what makes them worth a doctor.
Use the number as a mirror, not a verdict. It is measuring your habits more reliably than it is measuring your sleep.
