Almost every case of this is one of four settings, and they are worth working through in order rather than jumping to the conclusion that the sensor has failed. The first one accounts for more cases than the other three combined.
Work through these in order
- 1
Turn Wrist Detection back on
On the iPhone: Watch app, Passcode, Wrist Detection. This is the single most common cause and the least obvious.
Turning it off disables heart rate measurement entirely, and sleep tracking depends on heart rate. People disable it because they dislike the watch locking or because of a tattoo interfering with detection, then lose sleep tracking without connecting the two.
- 2
Confirm sleep tracking is actually enabled
On the iPhone: Health app, Browse, Sleep, then check Sleep Focus and the tracking options. On the watch: Settings, Sleep. Both need to be on, and a watchOS update occasionally resets one of them.
- 3
Set a Sleep Schedule
In the Health app under Sleep. The watch uses your schedule to know when to expect sleep, and the Sleep Score compares your duration against the goal you set here. Without a schedule, tracking is less reliable and the score has nothing to measure against.
- 4
Charge to at least 30 percent before bed
Build a charging window into your routine, typically while showering or during the evening. A watch that dies overnight produces a partial night, which is worse than no night at all for anything building a baseline.
- 5
Fit the band properly
Snug, with the back in contact with your skin, and not sliding around as you move. A loose band is the usual reason you get time in bed with no stage breakdown: the movement data is there and the heart rate data is not.
- 6
Restart both devices
Genuinely worth doing before anything more drastic, and it resolves a surprising share of cases where everything looks correctly configured.
- 7
Update watchOS and iOS
Sleep tracking has changed substantially across recent releases, and Sleep Score requires watchOS 26 or later. A mismatch between watch and phone versions causes its own problems.
If everything is set correctly and it still fails
Check Sensor Diagnostics on the watch under Settings, General, About. If it reports errors, that is a hardware conversation with Apple rather than a settings problem.
Before that, one more thing worth ruling out: check whether another app has been writing sleep data to Apple Health. When two sources write overlapping sleep, Health picks a priority order, and the result can look like your watch has stopped tracking when in fact its data is being superseded. The data source priority list is in Health under your profile.
What to expect once it works
A Sleep Score arrives each morning on watchOS 26 or later, explained in the Apple Watch Sleep Score explained. The Vitals app needs about seven nights before it can classify anything, and sleep stages appear in the Health app the following morning rather than instantly.
For getting more out of the tracking once it works, see the Apple Watch sleep tracking guide. If HRV specifically is the thing that is blank, see Apple Watch HRV not showing.