What Your Sleep Is Really Telling You
Sleep trackers have flooded the market, but most people are reading the numbers wrong. A short guide to what actually matters.
There is a strange new ritual at the start of the day: opening an app to see how you slept. The app delivers a verdict — a score, a graph, a phrase like "recovery: low." Some people then feel tired all day because the app told them they would be.
This is not a small problem. A growing body of research suggests that worrying about sleep is itself one of the most reliable ways to sleep badly. So before you check your score tomorrow, it is worth knowing what these devices actually measure — and what they don't.
What trackers do well
Wrist-worn devices are reasonably good at one thing: estimating how long you were asleep versus awake. They use heart rate, movement, and sometimes skin temperature, and the algorithms have improved a great deal in the last few years.
What they do badly
They are not good at measuring sleep stages. Distinguishing light sleep from REM sleep from deep sleep requires brain wave data — the kind you get in a lab, not on your wrist. Most consumer devices guess, and the guesses can be wildly off.
The "deep sleep" number you see in the morning is, in most cases, a polite estimate.
What to watch instead
If you want a useful signal, focus on three things you can feel:
- How long it took you to fall asleep. Consistently under 20 minutes is good.
- Whether you wake up feeling rested. Not perfect — rested.
- Daytime energy without coffee. This is the real test.
The numbers are interesting. Your own body is the better instrument.
