Metric guide
Your watch is better at timing sleep than naming every stage
The useful version
Focus on duration, timing and repeated patterns. Do not chase one night of stage percentages.
What the watch measures
Garmin watches infer sleep from movement, heart rate and changes in beat-to-beat timing. Those are useful signals, but they are indirect.
A clinical sleep study uses additional measurements, including brain activity. A watch therefore estimates stages rather than observing them directly.
Scores versus the underlying night
A single score compresses duration, stress, movement and estimated stages into an easy verdict. That convenience can hide which input actually changed.
Open the timeline first. Bedtime, wake time, gaps and total duration are usually easier to interpret than a small score change.
Using sleep in training decisions
One poor night does not erase fitness. Repeated short sleep, a clear downward trend or poor sleep alongside symptoms is more meaningful.
Use the data as context for perceived fatigue and session quality, not as permission that must be granted before every workout.
Go one step further
Questions
What people usually ask
How does Garmin estimate sleep?
A watch combines movement and optical heart-rate signals to estimate when you slept and how the night was divided. It does not measure brain activity like a clinical sleep study.
Should I trust the sleep stages?
Treat the individual stage minutes cautiously. Consumer wearables tend to be more useful for sleep timing and duration than for precisely separating light, deep and REM sleep.
Should a poor score cancel hard training?
Not automatically. Consider how you feel, whether several nights are trending down, and whether the planned session can be adjusted. One noisy score is weak evidence.