This can be caused by wrist or arm movement during sleep, leading your watch to fail to recognize that you were sleeping, and thus will not record sleep data. To prevent inaccurate readings, try wearing the watch on your non-dominant hand while sleeping (on your left hand if you are right-handed, and vice versa).
Why is the sleep data missing?
If you sleep less than 4 hrs, your watch may not be able to detect this as sleep time, and will not record this sleep data in this instance.
If the watch is worn too loosely, it may fail to record sleep data. Make sure that you wear the device correctly.
When you are traveling, and sleeping on when in transit, such as on the metro, or on a bus or train, the movement of the train or vehicle may affect the recorded sleep data.
Your watch determines whether it is being worn through a built-in PPG sensor, which reflects light off your skin. The built-in sensor in the watch usually detects light reflected from human skin at different wavelengths. When the wearable device is placed on surfaces such as a desk, sofa, or quilt with the sensor facing down, light reflected from these surfaces may be similar to that from human skin. In this case, the watch may think that you are wearing the device, and start recording sleeping data.
If you turned over or accidentally touched the wearable device screen multiple times while sleeping, the device may exit Sleep mode and record false wake up times, based on the detected movements.
If there is minimal movement after waking up, the wearable device may remain in Sleep mode, and fail to record the actual wake up time.
The accuracy of sleep monitoring is closely related to how you wear the watch and your wrist movement.
Make sure that you keep the wearable device facing upward and wear it slightly back on the wrist. Do not wear the wearable device too tight or too loose to avoid inaccurate sleep data.
When you do not use your wearable device for a while, turn it to its side and place it on a flat surface to minimize any inaccurate sleep data recorded. Power off your wearable device if you do not use it for a long time.