My tracker says 38 minutes of deep sleep and I feel fine — is that number worth anything?

Consumer wearables infer stages from movement, heart rate and heart rate variability, and they are far better at telling asleep from awake than at telling one stage from another. Every validation comparison I have read against proper polysomnography shows stage level agreement is substantially worse than the overall sleep and wake agreement, and deep sleep in particular is the one they get wrong most. So treat the absolute minutes as decorative and the night to night trend as weakly informative. Do not buy a mattress because of a stage graph.

I spent about eight months chasing the score and ended up sleeping worse, because I would look at the number before I was properly awake and then feel tired on the strength of it. Two friends have described exactly the same loop. I took the ring off for a month, slept the same, felt better, and now wear it a week a month at most.

I have two years of data and the thing that convinced me the stage numbers are soft is that they move for reasons that have nothing to do with sleep quality. Dropping my bedroom from 20 to 17 degrees moved my reported deep sleep up by about a third within a week, which sounds like a great result until you notice that sleeping in a different position also moves it, and so does wearing the ring tighter. If a number responds that much to how you wear the sensor, it is not measuring what the label says.

I will defend trends, though not absolutes. I could not have told you that two drinks in the evening cost me something without the data, but the pattern was so consistent across dozens of nights that I believe the direction even if the minutes are made up. Same with a late heavy meal. Use it as a diary that fills itself in, not as a diagnostic.

You feel fine, you function, you do not need caffeine to get through the afternoon. That is the outcome measure. The ring is measuring a proxy for it badly.