Suunto AMBIT3 SPORT 2.0 User Manual page 77

You need to perform three tests while you are feeling well
recovered and have no accumulated recovery time to calibrate the
test algorithm.
The calibration tests do not need to be done in sequence.
During the calibration period, you may get results that are higher
or lower than expected. These results are indicated in the watch as
your estimated recovery status until the calibration is completed.
To perform a sleep recovery test:
1. Put on your heart rate belt and ensure the electrodes are moist.
2. Press [Start Stop], scroll to RECOVERY with [Start Stop], and
select with [Next].
3. Scroll to SLEEP TEST and select with [Next].
4. If you want to cancel the test, press [Back Lap].
5. When you are ready to get up in the morning, end the test by
pressing [Start Stop] or [Next].
When the test is ended, scroll through the results with [Next] and
[Back Lap].
7:45
test
duration
85%
Fully
recovered
44
Lowest HR
77